Include binstubs generated by Rails 4

Commit b516ecc added `bin/` to the .gitignore file. Now that Discourse
runs using Rails 4 by default, however, we should include the binstubs
generated by `rake rails:update:bin` in version control as this is the
recommendation of the Rails core team. Additionally, for those wishing
to deploy Discourse to Heroku, these binstubs are actually mandatory
according to [this article](https://devcenter.heroku.com/articles/rails4).

Other binstubs can continue to be ignored.

Signed-off-by: David Celis <me@davidcel.is>