Inherit from Capybara::Selenium::Driver

@sbonebrake I think the general approach for driver.rb is wrong. Instead we should start with `lib/capybara/selenium/driver.rb` and then only change what's necessary.

- appium supports web automation so in theory someone could use .visit and it'd work.
- this also fixes #6 which is caused from our driver diverging from the selenium driver
you're right. fixed.