Fix tests on Node 12

Node 12 and below don't have the `fs.rmSync` or `fs.rm` functions, and
has limited support for removing a directory recursively. Instead we
use `del`, which we include as a dependency anyway.

build/run-tasks.js

@@ -1,4 +1,5 @@
 const colour = require('nodemon/lib/utils/colour')
+const del = require('del')
 const fs = require('fs')
 const fse = require('fs-extra')
 const path = require('path')
@@ -25,8 +26,7 @@ function runTasks () {
 
 function clean () {
   // doesn't clean extensions.scss, should it?
-  fs.rmSync('public', { recursive: true, force: true })
-  fs.rmSync('.port.tmp', { force: true })
+  del.sync(['public/**', '.port.tmp'])
 }
 
 function sassExtensions () {