Use Phabricator

Summary:
Previously, I was just pushing stuff directly to origin/master.
I see we have a phabricator repository set up for Cassowary now, so let's make
`.arcconfig` work with that.

Test Plan: Run `arc diff` and see a diff get created!

Reviewers: bobo, edmond

Reviewed By: bobo

Differential Revision: https://phabricator.chronosresearch.com/D70