[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