Fix "not well-formed" error in Firefox.

Firefox requires the mime type to be set on the XMLHttpRequest used to
load javascript modules (see http:https://stackoverflow.com/a/677943/1005455),
otherwise it spits a "not well-formed" error that pollutes the console.
I'm patching Yabble directly.

utils/yabbler/yabble.js

@@ -305,6 +305,7 @@
 
  var xhr = this.XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP');
  var moduleUri = resolveModuleUri(moduleId);
+ xhr.overrideMimeType("application/json");
  xhr.open('GET', moduleUri, true);
  xhr.onreadystatechange = function() {
  if (xhr.readyState === 4) {