Send GitHub HipChat messages to another room

Summary: The consesus seems to be at least that GitHub commit notifications are noisy enough that we want them in a separate room. I've left the Phabricator notifications as-is (and team-specific commits are still sent to their respective rooms).

Test Plan: Crossed fingers.

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D11406