Don't use __DIR__ for PHP 5.2 compatibility

`__DIR__` constant is available since PHP 5.3 (http://php.net/manual/en/language.constants.predefined.php), while project's README claims compatibility with 5.2. I got tripped by this when trying to activate the plugin on my 5.2.17 server, so I made this fix to resolve it.

Feel free to propose a different solution to this problem. I'm not a PHP expert so I don't know if it won't cause problems for someone else; it works fine for me as of now.