Addd rspec-rerun to help travisci testing

See following for reference:

* https://github.com/dblock/rspec-rerun
* https://github.com/jonleighton/poltergeist/blob/master/README.md
* https://github.com/jonleighton/poltergeist/blob/master/.travis.yml
* https://github.com/jonleighton/poltergeist/blob/master/Rakefile