MB-6876:  revert to usintg "branch-20" for libconflate, to help isolate build failure

Change-Id: Ie667d2c703e785bbcf26e1e1c5d87c4ce9c99d35
Reviewed-on: http://review.couchbase.org/25457
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>