Update phantomjs gem

On mavericks the phantomjs gem cannot find new
versions of homebrew installed phantomjs binaries.
Later versions of the phantomjs gem install their own into
/Users/:user/.phantomjs.

This will fix this error
"Phantomjs does not appear to be installed in
/Users/:user/.phantomjs/1.9.7/darwin/bin/phantomjs, installing!"
Merge pull request #316 from alphagov/techdebt/update-phantomjs

Update phantomjs gem