Update current.py

JPG causes a KeyError: u'JPG' in 

sorl/thumbnail/base.py", line 196, in _get_thumbnail_filename
  EXTENSIONS[options['format']])

because there is JPEG used instead of JPG:

base.py:
EXTENSIONS = {
    'JPEG': 'jpg',
    'PNG': 'png',
}
Merge pull request #332 from g10f/patch-1

KeyError: u'JPG'