Install phantomjs

Summary:
Currently, phantomjs is already installed a linter dependency.

It's now exposed as a binary, per T572#18704, so developers can use it for tests purpose.

For static websites deployment, we can also use it to generate a screenshot of the homepage.

Test Plan: phantomjs

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: http://devcentral.nasqueron.org/D1759