Allow `FLEXIBLE_IMAGE_SIZES` to be a list of integers.
Allow `FLEXIBLE_IMAGE_SIZES` to be a list of integers.

lewiscollard/django-flexible-images,danielsamuels/django-flexible-images,lewiscollard/django-flexible-images,danielsamuels/django-flexible-images,danielsamuels/django-flexible-images,lewiscollard/django-flexible-images