Use a more up-to-date Phantomjs gem.

Switch to https://github.com/colszowka/phantomjs-gem for the Phantomjs gem.
It installs a more recent version of the binaries (1.8 vs 1.6), and now
supports the .run() API that teabag uses.
Merge pull request #41 from jabr/change-phantomjs-gem

Use a more up-to-date Phantomjs gem.