Address issue with SubfieldBase on Django 1.9

[paltman]

This is stemming from latest version of django-jsonfield and will apply pretty widely to Pinax projects/apps.

[JPWKU]

I read up on this so I thought I'd write here for posterity.
I browsed the django repository and found this
https://github.com/django/django/blob/aaacaeb0963c406c4fe6f68c6ae51f4a65878250/django/db/models/fields/subclassing.py#L21 ... In fact the sublassing.py module no longer exists. Here is how it was used in django-jsonfield:
django-jsonfield copied that file over into their repo directly here https://github.com/JPWKU/django-jsonfield/blob/master/jsonfield/subclassing.py and used it here https://github.com/JPWKU/django-jsonfield/blob/master/jsonfield/fields.py#L53

[paltman]

I was testing against Django master which has already flipped over to 1.10. The existing django-jsonfield used by Pinax works with 1.9.

However, in the process, I tested jsonfield against master and it worked just fine. We might should re-evaluate the standard Pinax library we use for JSON fields. They seem to have an interchangeable public API.