Deprecation warnings on Django 1.10

[mdentremont]

After updating to Django 1.10, I'm seeing a deprecation warnings which originate from django-sortedm2m:

...
/Users/<username>/.virtualenvs/<proj>/lib/python3.5/site-packages/sortedm2m/fields.py:334: RemovedInDjango20Warning: Usage of ForeignObjectRel.to attribute has been deprecated. Use the model attribute instead.
  to_model = self.rel.to
/Users/<username>/.virtualenvs/<proj>/lib/python3.5/site-packages/sortedm2m/fields.py:354: RemovedInDjango20Warning: Usage of field.rel has been deprecated. Use field.remote_field instead.
  if self.rel.to == RECURSIVE_RELATIONSHIP_CONSTANT or to_object_name == klass._meta.object_name:
/Users/<username>/.virtualenvs/<proj>/lib/python3.5/site-packages/sortedm2m/fields.py:354: RemovedInDjango20Warning: Usage of ForeignObjectRel.to attribute has been deprecated. Use the model attribute instead.
  if self.rel.to == RECURSIVE_RELATIONSHIP_CONSTANT or to_object_name == klass._meta.object_name:
/Users/<username>/.virtualenvs/<proj>/lib/python3.5/site-packages/sortedm2m/compat.py:37: RemovedInDjango20Warning: Usage of field.rel has been deprecated. Use field.remote_field instead.
  db_constraint=field.rel.db_constraint)
/Users/<username>/.virtualenvs/<proj>/lib/python3.5/site-packages/sortedm2m/fields.py:360: RemovedInDjango20Warning: on_delete will be a required arg for ForeignKey in Django 2.0. Set it to models.CASCADE on models and in existing migrations if you want to maintain the current default behavior. See https://docs.djangoproject.com/en/1.10/ref/models/fields/#django.db.models.ForeignKey.on_delete
  **get_foreignkey_field_kwargs(self))
...

Most of these look fairly minor, but it just might be slightly annoying to make the changes work across older versions of Django.

[gregmuellegger]

To make things compatible with older Django versions, I usually place a helper function in compat.py. Would you be willing to provide a PR to fix the deprecation warnings?

[mdentremont]

@gregmuellegger Unfortunately I've moved on to a different company which doesn't use Django - so I wouldn't have any way to test this against an application anymore :(

[jharrell]

#104 seems to resolve these deprecation warnings