fixed bug, brew threw an error when upgrading phantomjs if it was already up to date.  Now it checks if it is up to date before upgrading

Summary: stopped brew from updating phantomjs if it is already up-to-date

Test Plan: Ran make again w and w/o phantomjs installed to verify that it still worked

Reviewers: csilvers, jlfwong

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D5001