HHVM Compatibility
After adding [HHVM to Travis](https://travis-ci.org/cboden/Ratchet/jobs/16842567) there's only one failing test. 

My first guess is the ini value `session.serialize_handler` is not set to the same default value in HHVM that it is in PHP. See [SessionProvider](https://github.com/cboden/Ratchet/blob/hhvm/src/Ratchet/Session/SessionProvider.php#L59)
