Herald - tweak accepted differential revision check slightly

Summary: use the loadReviewedBy function, which seems to do what we want -- returns a reviewer IFF the last thing was an accept

Test Plan: i believe in the power of loadReviewedBy

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran, aarwine

Differential Revision: https://secure.phabricator.com/D7903