Fixed issue with workers not unregistering as they should

We've hit this problem in production when there were tons of registered workers (thousands) while in reality there were 2 Clojure workers. Turned out that each time Clojure process called resque-clojure.supervisor/stop and exited the worker would still be left registered in Redis DB. This caused problems to how Ruby implementation of Resque client tries to clean up old workers - it started taking more than 30 seconds to start new worker with all these registered but non existing workers.

It looks like this is just a simple oversight, as obviously @watched-queues has the info necessary to unregister workers on stoppage.