Add phpunit.xml to .gitignore

[The documentation](http://symfony.com/doc/current/book/testing.html#phpunit-configuration) recommends ignoring `phpunit.xml`, and only committing `phpunit.xml.dist` in version control. However, the `.gitignore` that comes with the Symfony Standard distribution does not contain this file.

This PR adds `/app/phpunit.xml` to `.gitignore`.