Add transactionID to maniphest.gettransactions output

Summary:
This commit adds the transactionID field to manphest.gettransactions, to
satisfy the request in T10327

Test Plan: Call the maniphest.gettransactions endpoint, verify transactionID is present

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: chad, Korvin

Differential Revision: https://secure.phabricator.com/D15250