Update poltergeist to support phantomjs 2.0

In response to [my issue](https://gitlab.com/gitlab-org/gitlab-development-kit/issues/36) on the GDK.
PhantomJS 1.8 was not in the repo's anymore. Only poltergeist needed it, but they support PhantomJS 2.0 since 1.6.0. (next release since 1.5.1) [Their CHANGELOG](https://github.com/teampoltergeist/poltergeist/blob/master/CHANGELOG.md).

Tested locally with Ruby 2.1.5. As this took ages, I haven't tested further.

See merge request !403