Swapped out Capybara javascript drivers

Major reasons:

- Selenium driver doesn't work with Travis-CI
- Webkit driver is faster, and has more features

See https://github.com/thoughtbot/capybara-webkit
for more information about the new driver
Start an XVFB server on Travis builds

xvfb is required to run headless feature specs -- Travis-CI simply requires an xvfb process to be running in the background before the tests run.