Rename accept/dismiss modal methods.

`Capybara::Session` has an `accept_alert` method, among others, that leverage the `accept_modal` and `dismiss_modal` methods on the driver. Right now those are missing so Capybara is falling back to the Selenium implementation, which calls `switch_to` and breaks. From the comments in the appium lib it seems that these methods were intended to replace the accept/dismiss modal methods instead of being new.