Use phpunit.xml.dist instead of phpunit.xml

[Best practice how to ship PHPUnit configuration](http://www.testically.org/2010/08/24/best-practice-how-to-ship-phpunit-configuration/) - a view shared by [Sebastian Bergman](https://github.com/sebastianbergmann/phpunit/blob/master/phpdox.xml.dist), [Symfony](https://github.com/symfony/symfony/blob/master/phpunit.xml.dist) and many others.