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.
find_modal isn't part of the Capybara API - because of the proactive/reactive nature of dealing with modals Capybara doesnt really provide anything for dealing with them, other than the external API definition