Fix the tests

@liveh2o @localshred 
This was added to ensure server always exits. Before adding it, we would have constant hanging RPC servers, presumably because the main thread was waiting for a child thread to finish. Adding `exit 0` basically told the main thread not to wait for child threads.

After chatting with @quixoten, we think the same thing can be accomplished simply by moving the `exit 0` to *bin/rpc_server.rb*. This keeps the exit where the CLI code is invoked and out of the code paths that are being invoked by the specs.