Support for Django 2.2

[john-athena]

Seems like the library is not supported in Django 2.2.

File "/home/repo/venv/lib/python3.8/site-packages/sortedone2many/fields.py", line 277, in contribute_to_related_class
    if not self.rel.is_hidden() and not related.related_model._meta.swapped:
AttributeError: 'OneToManyField' object has no attribute 'rel'

Seems like rel has been removed: https://docs.djangoproject.com/en/2.0/releases/1.9/#field-rel-changes

I could confirm that changing self.rel to self.remote_field "worked". That said, 'rel' seems to be prevalent in multiple places, so this single line patch isn't enough.