chore(package): update uglify-js to version 2.7.4

https://greenkeeper.io/

package.json

@@ -12,6 +12,6 @@
     "clean-css": "3.4.20",
     "less": "2.7.1",
     "stylus": "0.54.5",
-    "uglify-js": "2.7.3"
+    "uglify-js": "2.7.4"
   }
 }