Don't run requirements gate check job on stable/.

At least until we figure out how we want to handle that
in the requirements repo.

Change-Id: I595078cfe63098062538c2587e3c229131bef55a
Reviewed-on: https://review.openstack.org/26760
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins