Don't allow IPv6 for mock server

The test suite tries to connect to 127.0.0.1...

Change-Id: Ib55f28ed336909946bf522ebe1d1e60acbb3cbd7
Reviewed-on: http://review.couchbase.org/31625
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>