Fix signature of six.with_metaclass.

In six 1.4, the signature of `with_metaclass` has changed from `with_metaclass(meta, base=object)` to `with_metaclass(meta, *bases)`, allowing for multiple bases. This breaks django-jsonfield, which uses the `base` kwarg name explicitly. Since Django 1.6b3 updated the bundled `django.utils.six` to version 1.4.1, this also makes jsonfield broken with Django 1.6b3.

This pull request fixes django-jsonfield's use of `six.with_metaclass` to not specify the base as a kwarg, but rather as a positional arg, making it compatible with both older and newer versions of Django/six.