Update requirements.txt
Created by: tdviet
Require python-six >= 1.4.0 due to method add_metaclass(metaclass). By the way, openstack havana require six <1.4.1, so 1.4.0 is the only suitable version for Havana
Created by: tdviet
Require python-six >= 1.4.0 due to method add_metaclass(metaclass). By the way, openstack havana require six <1.4.1, so 1.4.0 is the only suitable version for Havana