Bundlerize

Merge from `bundle gem` templates

Background:
As a gem user, when a gem is updated, I'd like to check the diffs on GitHub like https://github.com/rails/rails/compare/v4.1.6...v4.1.7 
To do so, git tag is useful. Bundler's `rake release` task makes us easy to create git tag.
In addition, it would be helpful if you set `Source Code URL` with `https://github.com/EasyPost/easypost-ruby` on https://rubygems.org/gems/easypost