chore(development): fix npm start file watching (#3922)

A invalid argument (`--w`) was being passed to the babel cli in the grunt task `shell:babel`. This argument has been changed to `--watch` to get the babel cli to watch files for changes as intended.

build/grunt.js

@@ -452,7 +452,7 @@ module.exports = function(grunt) {
     },
     shell: {
       babel: {
-        command: 'npm run babel -- --w',
+        command: 'npm run babel -- --watch',
         options: {
           preferLocal: true
         }