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

Fixes #58: Memory leak on JRuby.
Kill thread specifically when testing #send_metrics
Factor metrics worker thread start/stop (fixes 1.9.2 TypeError)
Add pending worker specs
Use earlier version of Rubygems to avoid Bundler fatal error on Travis.

https://github.com/bundler/bundler/issues/2784
https://github.com/freerange/mocha/commit/66bab2a8f4e7cd8734bf88e6f32157c0d5153125
Use mutex when duplicating metrics Hash. Fixes #60

Conflicts:
	CHANGELOG.md
Update changelog
Fixed bug with missing stddev stats
Add entry to changelog