Upgrade to JSON-API RC3 + latest version of tobscure/json-api

Note: npm source for ember-json-api changed to a fork, but I still had
to apply a custom change to get polymorphic relationships to work (see
https://github.com/kurko/ember-json-api/pull/71#issuecomment-85257281)

ember/app/models/post-stream.js

@@ -182,7 +182,7 @@ export default Ember.ArrayProxy.extend(Ember.Evented, {
     if (post) {
       item.setProperties({
         content: post,
-        component: 'discussion/post-'+post.get('type')
+        component: 'discussion/post-'+post.get('contentType')
       });
     }
     return item;