Add six back to setup.py

setup.py

diff --git a/setup.py b/setup.py
index ad14930..8f9550d 100644
--- a/setup.py
+++ b/setup.py
@@ -28,6 +28,7 @@
         'flake8!=2.5.3',
         'autopep8>=1.3',
         'pyyaml',
+        'six',
     ],
     entry_points={
         'console_scripts': [