Fix an exception when viewing Phriction diffs.

Summary:
D4117 (which is otherwise awesome :)) requires you to setMarkupEngine() and
Phriction's diff rendering wasn't changed to call that with a
PhabricatorMarkupEngine.

Test Plan: Went to a Phriction diff page and saw it render correctly.

Reviewers: epriestley, btrahan, vrana

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4132