Ignore phpunit.xml in Symfony2 applications

Symfony2 application sometimes use ".dist" file to distribute template configuration, and invite the developer to create a custom local copy from scratch.

Quoting the documentation (http://symfony.com/doc/current/book/testing.html#phpunit-configuration):
> Store the app/phpunit.xml.dist file in your code repository and ignore the app/phpunit.xml file.

You already done the same for ```parameters.yml``` and ```parameters.ini```, so I think that there are no drawbacks in including this one.

What do you think?