Add a git commit hash to perseus.js for easier subperseus debugging

Summary:
Adds the latest git commit hash to the second line of perseus.js
This way subperseus commits will have a revision attached, which can
be looked up in the case of debugging a subperseus.

Test Plan:
make
Open build/perseus.js
Check that the latest git commit hash is shown on the second line
Diff with the latest subperseus

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5492