gc_enable is not called if running with HHVM
gc_enable is not called if running with HHVM

Since HHVM doesn't support garbage collector (for obvious reasons), the following patch make Ratchet works with HHVM as well.
ctkjose/Ratchet,Nek-/Ratchet,b-viguier/Ratchet,KSDaemon/Ratchet,krageon/Ratchet,IRlyDontKnow/Ratchet,ratchetphp/Ratchet,vinod1988/Ratchet,Jumplead/Ratchet,cesarmarinhorj/Ratchet,sudarshan955/Ratchet,lyrixx/Ratchet,naroga/Ratchet,mbonneau/Ratchet,zjien/Ratchet