Make sure /usr/local/bin is before /usr/bin on the PATH.

Summary:
David Bieber was having trouble in that mac-setup install a new git
(via brew) but he was still using the system git.  The reason is that
/usr/bin was before /usr/local/bin on his PATH.  Make sure that
doesn't happen again.

Test Plan: Fingers crossed (I don't have a mac)

Reviewers: zach, davidbieber

Reviewed By: zach

Differential Revision: http://phabricator.khanacademy.org/D2583