image needs to be rgb, not cmyk

CMYK jpegs error out if you don't first convert to RGB:

```
  File "/home/chriscauley/src/django-crop-override-field/crop_override/admin.py", line 56, in save_model
    save_crops(obj, request)

  File "/home/chriscauley/src/django-crop-override-field/crop_override/admin.py", line 47, in save_crops
    c.save(buf, "PNG")

  File "/usr/local/lib/python2.7/dist-packages/PIL/Image.py", line 1452, in save
    save_handler(self, fp, filename)

  File "/usr/local/lib/python2.7/dist-packages/PIL/PngImagePlugin.py", line 530, in _save
    raise IOError("cannot write mode %s as PNG" % mode)

IOError: cannot write mode CMYK as PNG
```