libs3: update to latest git master of ceph/libs3

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Merge pull request #1917 from ceph/wip-da-update-libs3

libs3: update to latest git master of ceph/libs3

Reviewed-by: Loic Dachary <loic@dachary.org>