[composer] Require symfony 2.1.* instead of dev-master
[composer] Require symfony 2.1.* instead of dev-master

This allows using ratchet with a forked version of symfony.
sudarshan955/Ratchet,krageon/Ratchet,ctkjose/Ratchet,zjien/Ratchet,IRlyDontKnow/Ratchet,cesarmarinhorj/Ratchet,Nek-/Ratchet,lyrixx/Ratchet,mbonneau/Ratchet,b-viguier/Ratchet,vinod1988/Ratchet,KSDaemon/Ratchet,ratchetphp/Ratchet,naroga/Ratchet,Jumplead/Ratchet