Fixed filter-classname for sebastian\diff

The class SebastianBergmann\Diff is not existing, which results in an error, if the autoload of zf1 is used:

Fatal error: Uncaught exception 'PHPUnit_Framework_Error_Warning' with message 'include_once(SebastianBergmann/Diff.php): failed to open stream: Datei oder Verzeichnis nicht gefunden' in Zend/Loader.php on line 134.

Although this is caused by Zend_Loader, it should be fixed by using an existing class like SebastianBergmann\Diff\Diff