Specify poltergeist version

This will allow poltergeist to specify the Capybara verison. Currently
tests don't pass due to poltergeist throwing a nil exception. See
https://github.com/jonleighton/poltergeist/issues/320#issuecomment-29307753.
Merge pull request #359 from KevinSjoberg/degrade-poltergeist

Specify poltergeist version