bump django-extentions to latest 1.3.10
change model to use local copy of jsonfield

so the packaging requirements become simpler. This is to make
developer's lives easier because spaghetti requirements are terrible. I
did end up having to trade it for 'six', but 'six' is a much more
reasonable requirement.

closes #6
remove django_extensions as a requirement for testing
keep vendor'd file out of coverage
Merge pull request #7 from texastribune/no-dj-ext

Remove django_extensions as a hard requirement