Make ruby-version work with rbenv
Revised version of Learn/Vol buttons for signed out users.

* Show the 'Sign In' form as a modal instead of going to another page
* Show the Learn/Volunteer buttons in their own "RSVP" column rather
  than hijacking the Location column

This required some somewhat shady js layered on top of bootstrap's
  rather anemic modal. Check out sign_in.js.coffee for some fun.
Merge branch 'ruby-version' of https://github.com/fionatay/bridge_troll into fionatay-ruby-version