Update bundled gems

I ran `bundle update`. I think/hope this'll make it easier to upgrade to
Rails 4.
Specify Ruby 2.0.0

I've additionally specified 2.0.0-p247 (which is the version on
Heroku[1]) in .ruby-version even though that's not in the repo.

All the tests pass on my laptop so I'm hoping they'll pass on Travis and
that it'll all just work once deployed to Heroku.

[1] https://devcenter.heroku.com/articles/ruby-support#ruby-versions
Fix asset precompilation

Running `rake assets:precompile` failed on Heroku (and locally) with the
following error: "'fontawesome.less' wasn't found". This change (as per
the README[1]) fixes the problem in development and will hopefully do
the same on Heroku.

[1] https://github.com/seyhunak/twitter-bootstrap-rails/#icons