Bump djangorestframework from 3.3.2 to 3.9.1

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.3.2 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.3.2...3.9.1)

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

requirements.txt

@@ -4,7 +4,7 @@ Django==1.9.4
 django-autocomplete-light==3.1.1
 django-crispy-forms==1.6.0
 django-jsonview==0.5.0
-djangorestframework==3.3.2
+djangorestframework==3.9.1
 ipython==4.1.2
 ipython-genutils==0.1.0
 path.py==8.1.2