Convert string references to ::class

PHP 5.5.9 adds the new static `class` property which provides the fully qualified class name. This is preferred over using strings for class names since the `class` property references are checked by PHP.

app/Filters/SubscriptionFilter.php

@@ -11,7 +11,7 @@ class SubscriptionFilter extends FilterAbstract
      *
      * @var mixed
      */
-    protected $class = '\App\Models\Subscription';
+    protected $class = \App\Models\Subscription::class;
 
 
     /**