Bump djangorestframework from 2.4.8 to 3.11.2 in /requirements

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 2.4.8 to 3.11.2.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](https://github.com/encode/django-rest-framework/compare/2.4.8...3.11.2)

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

requirements/requirements.txt

@@ -19,6 +19,6 @@ PyYAML==3.11
 reportlab==2.7
 requests==2.8.1
 six==1.10.0
-djangorestframework==2.4.8
+djangorestframework==3.11.2
 django-ipware==1.1.6
 django-tinymce==2.6.0