Does not work with Django 2.0

[haakenlid]

django-url-filter uses some features that have been deprecated with django version 2.0

Usage of field.rel has been deprecated. Use field.remote_field instead.

This is used at least one place in django-url-filter.

https://github.com/miki725/django-url-filter/blob/master/url_filter/filtersets/django.py#L126

[haakenlid]

I've submitted a pull request to fix this. It passes tests for both django 1.8 and django 2.0 when I run tox locally.