Fix bug preventing concrete inheritance of models w GenericForeignKey
Fix bug with order of model field loading

django.db.models.base.ModelBase.__new__() iterates over the
fields on a model and calls field.contribute_to_class() on each.
ContentTypeSourceField.contribute_to_class() incorrectly
assumed that the field referenced by `source_field` has always
already been loaded.
Bump version to v1.0.15