set shard category for AR Job in rails 3

Change-Id: I29e57c3668059734ab852c984c6608840ffd5daf
Reviewed-on: https://gerrit.instructure.com/30774
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>