Build was failing with this message on Ubuntu 12.10:

  File "~/dev/env/local/lib/python2.7/site-packages/jsonfield/fields.py", line 50, in <module>
    class JSONFieldBase(six.with_metaclass(SubfieldBase, base=models.Field)):
TypeError: with_metaclass() got an unexpected keyword argument 'base'
Merge pull request #223 from ottenhoff/fix-build

Build was failing with this message on Ubuntu 12.10: