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/Models/Event.php

@@ -43,7 +43,7 @@ class Event extends Model
      */
     public function attachments()
     {
-        return $this->morphMany('App\Models\Attachment', 'attachable');
+        return $this->morphMany(\App\Models\Attachment::class, 'attachable');
     }
 
     /**