Bump djangorestframework from 3.9.1 to 3.11.2 in /requirements

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.9.1 to 3.11.2.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.9.1...3.11.2)

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

requirements/django.txt

@@ -38,6 +38,6 @@ django-bootstrap-form==3.2
 django-registration-redux==1.2
 django-bootstrap3-datetimepicker==2.2.3
 ##djangorestframework==2.4.4
-djangorestframework==3.9.1
+djangorestframework==3.11.2
 django-crispy-forms==1.4.0
 South==1.0.2