Change the ONBUILD to be as special of a snowflake as Ruby itself is

Since our ONBUILD already assumes a Gemfile, we also get to assume that `gem "rails"` is in it somewhere in some shape or form, which means that we don't need to be based on our image that already includes a version of Rails.
Merge pull request #5 from infosiftr/simpler-onbuild

Change the ONBUILD to be as special of a snowflake as Ruby itself is