Don't specify port for stun server

No idea why but, with it, when a client disconnects the datachannel
doesn't close.  This wasn't an issue prior to 359600d since the
configuration wasn't being passed to the peerconnection constructor.

This probably just makes the url invalid and falls back to a default in
libwebrtc.

proxy/config.js

@@ -30,7 +30,7 @@ Config.prototype.connectionsPerClient = 1;
 Config.prototype.pcConfig = {
   iceServers: [
     {
-      urls: ['stun:stun.l.google.com:19302']
+      urls: ['stun:stun.l.google.com']
     }
   ]
 };