[tor-relays] Make your obfsbridge faster by installing python-gmpy

Greetings, here is a tip if you are running an obfs3/scramblresuit bridge: If your bridge is feeling too slow or flooded by too many connections, you can help it by installing the python-gmpy package. If you do that, obfsproxy will use python-gmpy opportunistically to perform some crypto operations (modular exponentiations) much much faster. FWIW, in future packages of obfsproxy, the python-gmpy will be installed as a recommended package.