Use private branch of couchstore

Change-Id: I461be9003a5cab95b1ca5aa384e9876bf5a21a01
Reviewed-on: http://review.couchbase.org/27250
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>