Makes evaluation of `source_file` lazy
Makes evaluation of `source_file` lazy

This paves the way for future updates that will fix bugs related to
specs not reflecting updates to their source image fields.

pcompassion/django-imagekit,FundedByMe/django-imagekit,tawanda/django-imagekit,FundedByMe/django-imagekit,tawanda/django-imagekit,pcompassion/django-imagekit,pcompassion/django-imagekit