PHP compatibility fix

Summary: Replace __DIR__ with dirname(__FILE__) because __DIR__ is only available in PHP 5.3.0.

Test Plan: If this differential works, then it seems that everything is okay.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9957