Merge pull request #7962 from mozilla/greenkeeper/uglify-js-3.3.18

fix(package): update uglify-js to version 3.3.18

package.json

@@ -25,7 +25,7 @@
     "source-map": "0.7.2",
     "raven-js": "3.24.0",
     "timeago": "1.6.2",
-    "uglify-js": "3.3.16",
+    "uglify-js": "3.3.18",
     "underscore": "1.8.3"
   },
   "devDependencies": {