EventDispatcherInterface fix in Symfony 2.4

With Symfony 2.4 the debug.event_dispatcher service was made default in debug mode.(symfony/symfony@1e1835ef59424b0ce16d0a74e2854946d06ce486) This class (TraceableEventDispatcher) implements the EventDispatcherInterface and doesn't extend the default EventDispatcher.
So the __constructor method will throw an exception when using EventDispatcher and not EventDisptacherInterface.