Verify Rails 3 to 4 without ./bin works

Looks like it is good to go. Using new major release of Hatchet that fixed all sorts of compatibility issues with running interactive consoles. Split all of that code into a separate lib: https://github.com/schneems/repl_runner
Merge pull request #114 from heroku/schneems/rails4-no-bin

Verify Rails 3 to 4 without ./bin works
Merge branch 'master' of git://github.com/heroku/heroku-buildpack-ruby