PhantomJS is required to run JavaScript-dependent specs.
SSH into your Vagrant Virtual Machine if you're using one, then run the following commands to install PhantomJS:
curl -sL https://deb.nodesource.com/setup | sudo bash -
sudo apt-get install nodejs
npm install -g phantomjs
Only do this if you're not using Vagrant:
Follow installation instructions for Mac OS X: http://phantomjs.org/download.html