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.
I think we should do what's reasonable to get Capybara 2.4 support out; it will be nice to get caught up.

After the release, though, I think it makes sense to try and make the server act synchronously with commands. I actually thought this was already the case, and I can't think of a reason you'd really want another command to go through before the previous command was finished.