Merge pull request #5348 from mozilla/greenkeeper/uglify-js-3.0.1

fix(package): update uglify-js to version 3.0.1

package.json

@@ -12,6 +12,6 @@
     "clean-css-cli": "4.0.12",
     "less": "2.7.2",
     "stylus": "0.54.5",
-    "uglify-js": "2.8.22"
+    "uglify-js": "3.0.1"
   }
 }