explicitly restrict merge candidates scope shard for rails 3

Change-Id: I77eacd83c3feefeb76af4b252dc8f3b7f05afd24
Reviewed-on: https://gerrit.instructure.com/29042
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>