Use :gitub key, :tag key, and production group in Gemfile

Two Gemfile related commits. I've left them as two commits as I think they add different things.

The first adds github and tag keys. Tags are important if bundling against master as it can easily vary experience on different systems. I've left the rouge one without as I believe you're hoping master will be updated to fix #157.

The newrelic to production thing just stops newrelic slowing down development/test mode (it can be really annoying in my experience).