Be more specific when pointing to a tag for 1.8.1 hotfix branch

Change-Id: I0b16792a7f76110f439b596d25bf693751ccc910
Reviewed-on: http://review.couchbase.org/25348
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>