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/Model/helpdesk/Ticket/Ticket_ThreadOld.php

@@ -16,7 +16,7 @@ class Ticket_ThreadOld extends Model
 
     public function attach()
     {
-        return $this->hasMany('App\Model\helpdesk\Ticket\Ticket_attachments', 'thread_id');
+        return $this->hasMany(\App\Model\helpdesk\Ticket\Ticket_attachments::class, 'thread_id');
     }
 
     public function delete()