Bump django from 1.11.29 to 2.2.24 in /requirements

Bumps [django](https://github.com/django/django) from 1.11.29 to 2.2.24.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/1.11.29...2.2.24)

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

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

requirements/common.txt

@@ -1,5 +1,5 @@
 celery>4.4,<5.0
-Django==1.11.29
+Django==2.2.24
 django-appconf
 django-assets
 django-celery-beat<2.0