Fix comparison between Python Requirement objects.

pkg_resources.Requirement implements __eq__() but not __ne__(). There is
no implied relationship between __eq__() and __ne__() so we must negate
the result of == instead of using !=.

Change-Id: Ie83f87687f5666e6511265f2b9acdd42f38f1dd2
Reviewed-on: https://review.openstack.org/25974
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins