chore: use npm 7 workspaces instead of lerna bootstrap

It's faster - running `time npm i` shows a nice speed up:

lerna

```
npm timing command:install Completed in 183859ms
npm timing npm Completed in 184077ms
npm info ok

real	3m4.246s
user	2m48.066s
sys	0m58.948s
```

npm7 + workspaces

```
npm timing command:install Completed in 101724ms
npm timing npm Completed in 101914ms
npm info ok

real	1m42.075s
user	1m52.422s
sys	0m46.022s
```

The downside is it requires npm7 - it's bundled with node 15 but will
require users up upgrade if they are using node 14 or below.

.travis.yml

@@ -16,8 +16,8 @@ stages:
   - release
 
 node_js:
-  - 'node'
   - 'lts/*'
+  - 'node'
 
 os:
   - linux
@@ -27,6 +27,8 @@ os:
 before_install:
   # prevents windows error: npm ERR! ... git-sh-setup: file not found
   - if [ "$TRAVIS_OS_NAME" = "windows" ]; then export PATH=/c/PROGRA~1/Git/usr/bin:/c/PROGRA~1/Git/mingw64/libexec/git-core:$PATH ; fi
+  # workspaces require npm 7 or above
+  - npm install -g npm@latest
 
 script: npx nyc -s npm run test -- -- -- -t node --bail
 after_success: npx nyc report --reporter=text-lcov > coverage.lcov && npx codecov

package.json

@@ -3,7 +3,6 @@
   "version": "1.0.0",
   "description": "JS implementation of the IPFS UnixFS",
   "scripts": {
-    "postinstall": "lerna bootstrap",
     "reset": "lerna run clean && rm -rf packages/*/node_modules node_modules",
     "test": "lerna run test",
     "coverage": "lerna run coverage",
@@ -47,5 +46,11 @@
     "Diogo Silva <fsdiogo@gmail.com>",
     "jbenet <juan@benet.ai>",
     "Bernard Mordan <bernard@tableflip.io>"
-  ]
+  ],
+  "workspaces": [
+    "packages/*"
+  ],
+  "engines": {
+    "npm": ">=7.0.0"
+  }
 }