[Aphront] fix setter for AphrontCrumbsView
Summary:
The idiom is to return $this in all setters; I think this got missed in
AphrontCrumbsView by mistake.
Test Plan: Use an AphrontCrumbsView!
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, epriestley
Differential Revision: https://secure.phabricator.com/D1966