Bump djangorestframework from 3.1.0 to 3.9.1 (#17)

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.1.0 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.1.0...3.9.1)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -1,7 +1,7 @@
 Django==1.6.10
 South==1.0.2
 argparse==1.2.1
-djangorestframework==3.1.0
+djangorestframework==3.9.1
 ipaddr==2.1.11
 psycopg2>=2.6
 pyasn1==0.1.7