Restrict Capybara gem to 2.1.0. Version 2.2.0 does not work with Sauce connect

There has been a change to the Capybara gem, which now loads a local empty page to reset the session, rather than `about:blank`. When using Sauce Connect the local page isn't loaded and Capybara's internal assertion fails.

See issue 1198: https://github.com/jnicklas/capybara/issues/1198

The solution for now is to restrict it to Capybara 2.1.0.