Fix save_and_open_screenshot for 3rd drivers

The screenshot path will not start with `capybara/` for 3rd drivers like
poltergeist since they aren't on capybara itself.

See https://travis-ci.org/teampoltergeist/poltergeist/jobs/28737963#L534
Merge pull request #1347 from sobrinho/master

Fix save_and_open_screenshot for 3rd drivers