Pin six to a version that’s more likely to work

Everything is fine and happy locally and on Jenkins.

Getting a `cannot import name viewkeys` error on AWS.

Based on some Googling, it’s possible that this will fix it.

requirements.txt

@@ -19,6 +19,7 @@ pyexcel-xls==0.1.0
 pyexcel-xlsx==0.1.0
 pyexcel-ods3==0.1.1
 pytz==2016.4
+six==1.10.0
 wand==0.4.4
 gunicorn==19.6.0
 whitenoise==1.0.6  #manages static assets