MDL-50277 Javascript: Hardcode nodejs dependencies

In order to produce a consistent build across the board, we must hardcode
several package versions. We must additionally specify compatible
versions for any sub-dependency that we have a requirement upon.

In this instance, the only sub-dependency which has an effect upon the
overall output is grunt-contrib-uglify's dependency upon uglify-js.

Since change in uglify-js may lead to changes in the generated output, we
must specify a fixed version of that dependency. That version must be
compatible with the dependency signature for grunt-contrib-uglify or the
dependency manager will just download a different version.

This issue also bumps the version of uglify-js to a version which changes
the output. This is the latest version at time of commit.

package.json

@@ -3,9 +3,10 @@
     "private": true,
     "description": "Moodle",
     "devDependencies": {
-        "grunt": "~0.4.5",
-        "grunt-contrib-jshint": "~0.11.0",
-        "grunt-contrib-uglify": "~0.7.0",
-        "shifter": "~0.5.0"
+        "grunt": "0.4.5",
+        "grunt-contrib-jshint": "0.11.2",
+        "grunt-contrib-uglify": "0.9.1",
+        "shifter": "0.5.0",
+        "uglify-js": "2.4.22"
     }
 }