Mark renderActionDescription as abstract

Summary:
I got caught by this... my custom Herald action doesn't implement this method and now I am hitting this exception:

Call to undefined method HeraldHipChatNotificationAction::renderActionDescription()

/usr/src/phabricator/src/applications/herald/adapter/HeraldAdapter.php:896

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D13779