Changing references of StringIO to BytesIO
Changing references of StringIO to BytesIO

Closes #84

WGBH/django-versatileimagefield,respondcreate/django-versatileimagefield,WGBH/django-versatileimagefield,respondcreate/django-versatileimagefield,respondcreate/django-versatileimagefield,WGBH/django-versatileimagefield