Fix the tests

@liveh2o @localshred 
@tamird, we have not been able to find it either. I don't believe the issue is in the protobuf gem itself, but external gems that misbehave when running inside the rpc server. The `exit 0` forces the ruby process to shutdown when the rpc server is shut down rather than waiting for misbehaving threads in other gems that lock the process indefinitely. Moving it to `bin/rpc_server` seems to me a sufficient solution to mitigate the issue without impacting the behavior of the gem in general.