Add Flutie to Gemfile

* We use `body_class` and `page_title` view helpers in the application
  layout, which come from Flutie.
* We had previously removed Flutie because Flutie 1.x included lots of
  styles which have been superceded by Bourbon/Neat/Smash.
* Flutie 2.x is just the view helpers and we think `body_class` and
  `page_title` are valuable.