fix transaction test for ZK 3.5.0 (BadVersion message)

Signed-off-by: Raul Gutierrez S <rgs@itevenworks.net>
add zero command (set a znode's value to None)

Signed-off-by: Raul Gutierrez S <rgs@itevenworks.net>
Require Kazoo >= 2.0

Signed-off-by: Raul Gutierrez S <rgs@itevenworks.net>