Add reviewstats repo to gerritbot config

Change-Id: I4e6f4e043bf47ec7608b1330cee6709bdec3177d
Reviewed-on: https://review.openstack.org/35147
Reviewed-by: Anita Kuno <anteaya@anteaya.info>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Elizabeth Krumbach Joseph <lyz@princessleia.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins