MB-11128 Don't ever downgrade compat version.

Change-Id: Ied608db745081485e76697b4ef38c0677a6eed46
Reviewed-on: http://review.couchbase.org/37134
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>