Merge pull request #5781 from mozilla/greenkeeper/uglify-js-3.0.21

Update uglify-js to the latest version 🚀

package.json

@@ -24,7 +24,7 @@
     "stylus": "0.54.5",
     "raven-js": "3.16.0",
     "timeago": "1.5.4",
-    "uglify-js": "3.0.20",
+    "uglify-js": "3.0.21",
     "underscore": "1.8.3"
   },
   "devDependencies": {