Update to Capybara 2.4

Of course, just as we merged 2.3 support, Capybara 2.4 was released!

The entire suite is green with the exception of the new modal API. The biggest change is https://github.com/jnicklas/capybara/pull/1322. However, I've taken a first pass at implementing the required changes and it doesn't look too bad.
Thanks for the clarification, @twalpole. I see that now -- I had only looked at `within_window` and `switch_to_window`.

@jferris, inspiration for `find_modal` came from the Selenium `Driver` implementation: https://github.com/jnicklas/capybara/blob/master/lib/capybara/selenium/driver.rb#L254. If we had a handle to a `Document` we could use `synchronize` as `window_opened_by` does. I'm not sure if that's possible, or desired.