Allow symfony/event-dispatcher > 2.1.x

This enables having phpDocumentor as a requirement in a symfony 2.2
application.
The API for EventDispatcher did not change between 2.1 and 2.2

I ran the tests with v2.19 and v2.2.1 of the symfony/event-dispatcher and had the same result for both (1 error, 1 failure which appear unrelated to this change)