Add gem 'poltergeist'.

It is a capybara javascript driver using phantomjs
Use poltergeist as capybara js driver if phantomjs is found.

Firefox driver steal mouse, which is disturbing in development. And use
poltergeist in CI can remove the dependencies on X.
Remove X dependencies from .travis.yml
Merge branch 'use-poltergeist-in-test-by-default-when-phantomjs-is-found' of github.com:doitian/19wu into doitian-use-poltergeist-in-test-by-default-when-phantomjs-is-found