Fix Phabricator Fox to work with new transactions

Summary: Phabricator's rewrite of Differential made the POST data sent with feed.http-hooks rather inscrutable; I just wrote https://secure.phabricator.com/D8710 which makes the output bearable again, and this commit updates the handler to work with the new format.

Test Plan: Deployed, opened a Phabricator review, saw a message in HipChat.

Reviewers: brianmerlob

Reviewed By: brianmerlob

Differential Revision: http://phabricator.khanacademy.org/D8007