Use django custom six module

Try to use django's six instead of cutting out six entirely, as proposed by:
https://github.com/azavea/django-queryset-csv/pull/30
https://github.com/steventlamb/django-queryset-csv/commit/ab1bd3c0cb01148518887760e4fc290c622b0518
Merge pull request #32 from steventlamb/topic/use_django_six

Use django custom six module