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.