Allow anvil access to stable branches.

Change-Id: I756ce5cdc7fcbb37a74a4b71e87291d0726a0815
Reviewed-on: https://review.openstack.org/24657
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins