Update to latest libcouchbase (2.4.0)

Change-Id: Ic1b28cfe7d6eb050c6c37ac1be80c953823e490f
Reviewed-on: http://review.couchbase.org/40041
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Brett Lawson <brett19@gmail.com>