Show test coverage directly after running test

I don't see why this is necessary.

It looks like it was originally added to the main react repository because of HHVM compatibility issues with PHAR, but that should be fixed now: https://github.com/reactphp/react/pull/250#issuecomment-31076239

Assuming the HHVM compatibility issue is moot, see https://github.com/FriendsOfSymfony/friendsofsymfony.github.com/issues/39 for prior arguments on why I'd be against listing phpunit as a dev dependency.