Removes need to use babel-register (#4865)

* Removes need to use babel-register

- Adds watch to watch changes when running the test to regenerate
- Tests are now pure node 8

* Adds timing to helper.js

* Update contribution guide

* Adds inline sourcemaps generation to restore coverage

* nits

package.json

@@ -70,6 +70,7 @@
     "dev": "npm run build && node bin/dev",
     "lint": "flow && eslint --cache ./",
     "build": "babel src/ -d lib/ --copy-files",
+    "watch": "babel --watch src/ -d lib/ --copy-files",
     "pretest": "npm run lint",
     "test": "cross-env MONGODB_VERSION=${MONGODB_VERSION:=3.2.6} MONGODB_STORAGE_ENGINE=mmapv1 TESTING=1 jasmine",
     "coverage": "cross-env MONGODB_VERSION=${MONGODB_VERSION:=3.2.6} MONGODB_STORAGE_ENGINE=mmapv1 TESTING=1 nyc jasmine",