Ignore phpunit.xml

...as recommended in http://symfony.com/doc/current/book/testing.html#phpunit-configuration
minor #663 Add phpunit.xml to .gitignore (nicwortel)

This PR was merged into the 2.6-dev branch.

Discussion
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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.

This PR adds `/app/phpunit.xml` to the `.gitignore` that comes with the Symfony Standard distribution.

Commits
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9053b6b Ignore phpunit.xml