Added differential revision reviewers information to differential.getrevision conduit call.

Summary:
The reviewer information was not available for revisions before this.

Test Plan:
Tested with Conduit Console that correct reviewers were returned.

Reviewed By: jungejason
Reviewers: jungejason
Commenters: epriestley
CC: epriestley, tuomaspelkonen, jungejason
Differential Revision: 160