tox: use pylint 1.6.x for now
    
    FreeIPA is not yet compatible with pylint 1.7.1+. Enforce pylint 1.6.x
    until all issues have been addressed.
    
    Related: https://pagure.io/freeipa/issue/6874
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>