Bump django-filter from 1.1 to 2.4.0 in /requirements

Bumps [django-filter](https://github.com/carltongibson/django-filter) from 1.1 to 2.4.0.
- [Release notes](https://github.com/carltongibson/django-filter/releases)
- [Changelog](https://github.com/carltongibson/django-filter/blob/main/CHANGES.rst)
- [Commits](carltongibson/django-filter@1.1.0...2.4.0)

---
updated-dependencies:
- dependency-name: django-filter
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements/base.txt

@@ -2,7 +2,7 @@ requests==2.20.0
 boto3==1.9.159
 celery[redis]==4.4.0
 django-celery-results==1.1.2
-django-filter==1.1
+django-filter==2.4.0
 django==2.2.28
 django-storages==1.7.1
 djangorestframework-gis==0.14.0