Update gulpfile and README for gulp 4.x

Fixes for these gulp errors:
  - AssertionError [ERR_ASSERTION]: Task function must be specified
  - AssertionError [ERR_ASSERTION]: Task never defined: readme
  -   gulpjs/gulp#802
  - The following tasks did not complete: check-priv
    - Did you forget to signal async completion?
  - Error: watching conf.priv.js,completions.js,conf.js,actions.js,help.js,keys.js,util.js,gulpfile.js: watch task has to be a function (optionally generated by using gulp.parallel or gulp.series)

Update README to use local ./node_modules/.bin/gulp instead of global
gulp.

README.md

@@ -238,7 +238,7 @@ Building `surfingkeys-conf` requires a few dependencies to be installed:
 
 4. __Gulp Build/Install__
 	```shell
-	$ gulp install # OR "gulp build" to build to ./build/.surfingkeys without installing
+	$ ./node_modules/.bin/gulp install # OR "./node_modules/.bin/gulp build" to build to ./build/.surfingkeys without installing
 	```
 
 	This will build the final configuration file and place it at `~/.surfingkeys`.
@@ -260,14 +260,14 @@ Building `surfingkeys-conf` requires a few dependencies to be installed:
 		- __Linux__: `file:///home/{USERNAME}/.surfingkeys` (replace `{USERNAME}` with your username) 
 		- __macOS__: `file:///Users/{USERNAME}/.surfingkeys` (replace `{USERNAME}` with your username) 
 		- __Windows__: `file://%Homedrive%%Homepath%/.surfingkeys` (This is a guess, please correct me if I'm wrong)
-	- __IV.__ Hack Away! If you ever make a change to any of your configuration files in the future, simply run `gulp install` again and your new configuration will automatically be loaded by SurfingKeys.
+	- __IV.__ Hack Away! If you ever make a change to any of your configuration files in the future, simply run `./node_modules/.bin/gulp install` again and your new configuration will automatically be loaded by SurfingKeys.
 
 	</details>
 
 	<details>
 	<summary><strong>Option B</strong>: Manually copy/paste into the SurfingKeys configuration form</summary>
 
-	- __I.__ Copy the contents of `./build/.surfingkeys` (or `$HOME/.surfingkeys` if you ran `gulp install`)
+	- __I.__ Copy the contents of `./build/.surfingkeys` (or `$HOME/.surfingkeys` if you ran `./node_modules/.bin/gulp install`)
 
 	- __II.__ Open the SurfingKeys [configuration page](chrome-extension://mffcegbjcdejldmihkogmcnkgbbhioid/pages/options.html)
 

README.tmpl.md

@@ -93,7 +93,7 @@ Building `surfingkeys-conf` requires a few dependencies to be installed:
 
 4. __Gulp Build/Install__
 	```shell
-	$ gulp install # OR "gulp build" to build to ./build/.surfingkeys without installing
+	$ ./node_modules/.bin/gulp install # OR "./node_modules/.bin/gulp build" to build to ./build/.surfingkeys without installing
 	```
 
 	This will build the final configuration file and place it at `~/.surfingkeys`.
@@ -115,14 +115,14 @@ Building `surfingkeys-conf` requires a few dependencies to be installed:
 		- __Linux__: `file:///home/{USERNAME}/.surfingkeys` (replace `{USERNAME}` with your username) 
 		- __macOS__: `file:///Users/{USERNAME}/.surfingkeys` (replace `{USERNAME}` with your username) 
 		- __Windows__: `file://%Homedrive%%Homepath%/.surfingkeys` (This is a guess, please correct me if I'm wrong)
-	- __IV.__ Hack Away! If you ever make a change to any of your configuration files in the future, simply run `gulp install` again and your new configuration will automatically be loaded by SurfingKeys.
+	- __IV.__ Hack Away! If you ever make a change to any of your configuration files in the future, simply run `./node_modules/.bin/gulp install` again and your new configuration will automatically be loaded by SurfingKeys.
 
 	</details>
 
 	<details>
 	<summary><strong>Option B</strong>: Manually copy/paste into the SurfingKeys configuration form</summary>
 
-	- __I.__ Copy the contents of `./build/.surfingkeys` (or `$HOME/.surfingkeys` if you ran `gulp install`)
+	- __I.__ Copy the contents of `./build/.surfingkeys` (or `$HOME/.surfingkeys` if you ran `./node_modules/.bin/gulp install`)
 
 	- __II.__ Open the SurfingKeys [configuration page](chrome-extension://mffcegbjcdejldmihkogmcnkgbbhioid/pages/options.html)
 

gulpfile.js

@@ -37,7 +37,7 @@ gulp.task("gulp-autoreload", () => {
   let p
   const spawnChildren = function spawnChildren() {
     if (p) p.kill()
-    p = spawn("gulp", ["lint-gulpfile", "install", "watch-nogulpfile"], { stdio: "inherit" })
+    p = spawn("./node_modules/.bin/gulp", ["lint-gulpfile", "install", "watch-nogulpfile"], { stdio: "inherit" })
   }
   gulp.watch("gulpfile.js", spawnChildren)
   spawnChildren()
@@ -57,32 +57,16 @@ gulp.task("lint", () =>
     .pipe(eslint())
     .pipe(eslint.format()))
 
-gulp.task("check-priv", () => {
+gulp.task("check-priv", (done) => {
   try {
     fs.statSync("./conf.priv.js")
   } catch (e) {
     // eslint-disable-next-line no-console
     console.log("Creating ./conf.priv.js based on ./conf.priv.example.js")
     fs.copyFileSync("./conf.priv.example.js", "./conf.priv.js", fs.constants.COPYFILE_EXCL)
   }
-})
-
-gulp.task("build", ["check-priv", "clean", "lint", "readme"], () => gulp.src(paths.entry, { read: false })
-  .pipe(parcel())
-  .pipe(rename(".surfingkeys"))
-  .pipe(gulp.dest("build")))
-
-gulp.task("install", ["build"], () => gulp.src("build/.surfingkeys")
-  .pipe(gulp.dest(os.homedir())))
-
-gulp.task("watch", () => {
-  gulp.watch([].concat(paths.scripts, paths.gulpfile), ["readme", "install"])
-  gulp.watch(paths.readme, ["readme"])
-})
 
-gulp.task("watch-nogulpfile", () => {
-  gulp.watch([].concat(paths.scripts), ["readme", "install"])
-  gulp.watch(paths.readme, ["readme"])
+  done()
 })
 
 gulp.task("readme", () => {
@@ -154,4 +138,26 @@ gulp.task("readme", () => {
     .pipe(gulp.dest("."))
 })
 
-gulp.task("default", ["build"])
+gulp.task("build", gulp.series("check-priv", "clean", "lint", "readme"), (done) => {
+  gulp.src(paths.entry, { read: false })
+    .pipe(parcel())
+    .pipe(rename(".surfingkeys"))
+    .pipe(gulp.dest("build"))
+
+  done()
+})
+
+gulp.task("install", gulp.series("build"), () => gulp.src("build/.surfingkeys")
+  .pipe(gulp.dest(os.homedir())))
+
+gulp.task("watch", () => {
+  gulp.watch([].concat(paths.scripts, paths.gulpfile), gulp.series("readme", "install"))
+  gulp.watch(paths.readme, gulp.series("readme"))
+})
+
+gulp.task("watch-nogulpfile", () => {
+  gulp.watch([].concat(paths.scripts), gulp.series("readme", "install"))
+  gulp.watch(paths.readme, gulp.series("readme"))
+})
+
+gulp.task("default", gulp.series("build"))