Bump coffeescript to 2.x

After auditing for breaking changes [0] so that the output produced uses
ES2015 classes.

[0] https://coffeescript.org/#breaking-changes)

proxy/package.json

@@ -25,7 +25,7 @@
     "jasmine": "2.5.2"
   },
   "dependencies": {
-    "coffee-script": "^1.12.2",
+    "coffeescript": "^2.4.1",
     "wrtc": "^0.0.61",
     "ws": "^3.3.1",
     "xmlhttprequest": "^1.8.0"