Allow the metrics worker thread to shut down so it won't remain live in hot deploy scenarios.
Allow the metrics worker thread to shut down so it won't remain live in hot deploy scenarios.

Fixes #58: Memory leak on JRuby.

kylerippey/honeybadger-ruby,Workable/honeybadger-ruby,kylerippey/honeybadger-ruby,gtd/honeybadger-ruby,gtd/honeybadger-ruby,honeybadger-io/honeybadger-ruby,honeybadger-io/honeybadger-ruby,Workable/honeybadger-ruby