Skip CleanMediaCommandTest with Symfony 2.3
Skip CleanMediaCommandTest with Symfony 2.3

It uses `workspace` property that is private on Symfony 2.3.

nicolasricci/SonataMediaBundle,Soullivaneuh/SonataMediaBundle,jerome-fix/SonataMediaBundle,vyshkant/SonataMediaBundle,jerome-fix/SonataMediaBundle,nicolasricci/SonataMediaBundle,nicolasricci/SonataMediaBundle,vyshkant/SonataMediaBundle,sonata-project/SonataMediaBundle,OskarStark/SonataMediaBundle,OskarStark/SonataMediaBundle,hug963/SonataMediaBundle,hug963/SonataMediaBundle,OskarStark/SonataMediaBundle,hug963/SonataMediaBundle,nicolasricci/SonataMediaBundle,hug963/SonataMediaBundle,vyshkant/SonataMediaBundle,jerome-fix/SonataMediaBundle,Soullivaneuh/SonataMediaBundle,Soullivaneuh/SonataMediaBundle,OskarStark/SonataMediaBundle,Soullivaneuh/SonataMediaBundle,vyshkant/SonataMediaBundle,jerome-fix/SonataMediaBundle,Soullivaneuh/SonataMediaBundle,hug963/SonataMediaBundle,nicolasricci/SonataMediaBundle,vyshkant/SonataMediaBundle