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