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.
Looking at this a little more:

There's nothing on the C++ side which will prevent multiple commands being received and acted on, but the Ruby driver shouldn't ever do so. In `browser.rb`, it always waits for a response before returning from `command`, so unless the C++ command emits a response before it's actually finished, multiple commands can't execute simultaneously. Are you seeing behavior where that isn't the case?