Output a screenshot_url in rspec error messages

If `Capybara::Screenshot.url_base` is set it prepends that url to the screenshot filename.

eg. screenshots saved to `/var/www/screenshots` and
`Capybara::Screenshot.url_base = "http://localhost/screenshots/"`

This is useful if the directory holding screenshots is served from a webserver.
(When looking at errors in jenkins the screenshot url becomes a link making it easy to click on and view)

Otherwise it uses the `default screenshot_path`.