Stab in the dark to see if poltergeist works on travis

@NYULibraries/hydra When initially setting up this project we couldn't get the cucumber tests to run headlessly on Travis. But since we're no longer using VCR which caused complications and all our dependencies are up to date Poltergeist now seems to pass without a problem on Travis. This solves our issue of failed cukes due to Selenium screen size/responsive design stuff.