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)

src/Core/Models/Discussion.php

@@ -47,7 +47,7 @@ class Discussion extends Model
      *
      * @var \Flarum\Core\Models\Post[]
      */
-    protected $addedPosts = [];
+    public $addedPosts = [];
 
     /**
      * The user for which the state relationship should be loaded.
@@ -154,17 +154,6 @@ class Discussion extends Model
         return $this;
     }
 
-    /**
-     * Get a list of the posts that have been added to this discussion during
-     * this request.
-     *
-     * @return \Flarum\Core\Models\Post[]
-     */
-    public function getAddedPosts()
-    {
-        return $this->addedPosts;
-    }
-
     /**
      * Specify that a post was added to this discussion during this request
      * for later retrieval.