Upgrade to GitHub v3 API

Summary: GitHub dropped support for the v2 API today, which breaks login and registration. Use the v3 API instead.

Test Plan: Registered and logged in with GitHub. Verified process pulled email/photo/name/username correctly.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2726