Thread safety

Since I'm running geckoboard-push updates in sidekiq (which uses threads), I needed geckoboard-push to be thread-safe, which it obviously wasn't before (it was storing the api_key in a class variable). In my app, each Geckoboard push uses a different API key (the app user's key, not mine), so there would be a potential race condition.

This was a simple change. I basically just changed api_key and api_version to be instance variables, and updates tests accordingly.