Feature/ruby version pinned in gemfile

I noticed we are not pinning the Ruby version in the Gemfile, like we do in many projects (see https://devcenter.heroku.com/articles/ruby-versions). 

This PR pins the user's default Ruby to the top of the Gemfile. Perhaps this should be dependent on whether Heroku is being used? I figure there is no harm (that I know of) to pinning it in non-Heroku use cases.

Thoughts, @cade @travisr ?

:smile: :baby: