Update rackspace.php

consistency. use __DIR__

Author: WanderingZombie

lib/rackspace.php

@@ -5,4 +5,4 @@
  * @copyright 2013 Rackspace Hosting, Inc.
  * @license http://www.apache.org/licenses/LICENSE-2.0
  */
-require_once dirname(__FILE__).'/php-opencloud.php';
+require_once __DIR__.'/php-opencloud.php';