Patching use of deprecated django.utils.simplejson

I got the following error:
```
    DeprecationWarning: django.utils.simplejson is deprecated; use json instead.
    from django.utils import simplejson
```
Patch from https://github.com/joshuakarjala/sorl-thumbnail/commit/04e019507ff9841443ae0b252aed2dc2472743ff