Bump djangorestframework from 3.9.1 to 3.11.2

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>

Author: dependabot[bot]

requirements.txt

@@ -1,6 +1,6 @@
 Django>=1.7,<1.8
 argparse==1.2.1
-djangorestframework==3.9.1
+djangorestframework==3.11.2
 ipaddr==2.1.11
 psycopg2-binary>=2.6
 pyasn1==0.1.7