Switched from Selenium to Capybara-webkit for the capybara javascript driver.

Had to downgrade and lock Capybara to version 2.0.3 because of this bug, which makes Capybara-webkit fail with capybara 2.1.0 (the error was "undefined method `find_xpath' for #<Capybara::Webkit::Driver:..."):

https://github.com/thoughtbot/capybara-webkit/issues/499
Fixed acceptancte tests to work with capybara-webkit.