add six to installation requirements

setup.py

diff --git a/setup.py b/setup.py
index 3050118..852d8e1 100644
--- a/setup.py
+++ b/setup.py
@@ -32,6 +32,7 @@
         'autopep8>=1.1',
         'pyyaml',
         'simplejson',
+        'six==1.9.0',
     ],
     entry_points={
         'console_scripts': [