Add phantomjs install to setup script

Test Plan:
On OSX, Run `mac-setup.sh`, ensure phantomjs is installed at the end of it, in both the
case that it was installed beforehand and the case that it's already installed.

On Linux, Run `linux-setup.sh` instead.

Reviewers: csilvers

Reviewed By: csilvers

Maniphest Tasks: T1574

Differential Revision: http://phabricator.khanacademy.org/D4113