Replace grunt-wp-assets with grunt-filerev and grunt-filerev-assets

By writing to assets-manifest.json with grunt-filerev-assets, it's
no longer necessary to commit changes to lib/scripts.php when developing

package.json

@@ -31,7 +31,8 @@
     "grunt-contrib-uglify": "~0.2.4",
     "grunt-contrib-watch": "~0.5.3",
     "grunt-contrib-less": "~0.8.1",
-    "grunt-wp-assets": "~0.2.1",
+    "grunt-filerev": "~0.2.1",
+    "grunt-filerev-assets": "~0.3.0",
     "grunt-modernizr": "~0.5.1",
     "load-grunt-tasks": "~0.3.0"
   }