Update Asana Auth adapter for "gid" API changes

Summary: Ref T13453. The Asana API has changed, replacing all "id" fields with "gid", including the "users/me" API call result.

Test Plan: Linked an Asana account. Before: error about missing 'id'. After: clean link.

Maniphest Tasks: T13453

Differential Revision: https://secure.phabricator.com/D20899