build: Clean up grunt task and update grunt-contrib-watch

* Use slice instead of substr for clarity.
* Remove redundant (and potentially hazardous) __dirname prefix.
  Grunt already resolves pathnames relative to the Gruntfile
  by default. We don't prefix the save file.write() call with
  dirname either, so it wasn't a complete cover anyway, and it's
  simply not needed.

Change-Id: I5ff6d9c1d2d0a627bb7348674728a9ab96b4d685