MDL-52250 npm: remove uglify-js dependency/bump grunt-contrib-uglify

A) Remove direct dependency on uglify-js:

This dependency was added in MDL-50277, but it isn't quite correct, the
way npm installs dependencies means that grunt-contrib-uglify *could* use
'our' version if its satisifed. But does not *always* and especially does
not if the grunt-contrib-uglify requirement is higher than ours.

Long story short, in many cases this means that this is used:
 node_modules/grunt-contrib-uglify/node_modules/uglify-js
But this is not:
  node_modules/uglify-js

In MDL-50277 we thought we were making the version sticky, but in
reality we were just fixing previous problems by updating
grunt-contrib-uglify to a version which fixed a bug:

 https://github.com/gruntjs/grunt-contrib-uglify/issues/313

It seems the better solution will come with using npm shrinkwrap to lock
down depdencies.

B) Bump grunt-contrib-uglify to 0.11.0

This moves us to uglify-js ~2.6.0 - which doens't change any output
files.

package.json

@@ -6,8 +6,7 @@
         "grunt": "0.4.5",
         "grunt-contrib-jshint": "0.11.3",
         "grunt-contrib-less": "1.1.0",
-        "grunt-contrib-uglify": "0.9.1",
-        "shifter": "0.5.0",
-        "uglify-js": "2.4.22"
+        "grunt-contrib-uglify": "0.11.0",
+        "shifter": "0.5.0"
     }
 }