fix setup.py: add six as a requirement
fix setup.py: add six as a requirement

Signed-off-by: Amit Beka <feb9f1199bc383ad0266545392aa23bd5a177c94@gmail.com>

dolaameng/keras,daviddiazvico/keras,DeepGnosis/keras,keras-team/keras,nebw/keras,kuza55/keras,keras-team/keras,kemaswill/keras,relh/keras