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.
It's odd that Capybara doesn't handle this, as it generally does with nodes and windows. Does Capybara expect `find_modal` to return the correct instance immediately?