Travis builds: try to download phantomjs using npm's phantomjs-prebuilt package
Travis builds: try to download phantomjs using npm's phantomjs-prebuilt package

Allowing the phantomjs Ruby gem to download it often fails
because the bitbucket link being used hits a rate limit
which can cause build failures like this one:

https://travis-ci.org/pivotal/projectmonitor/builds/134245941

BuildingSync/projectmonitor,BuildingSync/projectmonitor,BuildingSync/projectmonitor,pivotal/projectmonitor,pivotal/projectmonitor,pivotal/projectmonitor,pivotal/projectmonitor,BuildingSync/projectmonitor