Use older version of capybara.

Ruby versions older than 1.9.3 can't use capybara > 2.0.3.

Travis-provided error:

   Gem::InstallError: capybara requires Ruby version >= 1.9.3.
   An error occurred while installing capybara (2.1.0), and Bundler
   cannot continue.
   Make sure that `gem install capybara -v '2.1.0'` succeeds before
   bundling.