Bump DRF to 3.9.1 to address WS-2019-0037

requirements.txt

@@ -7,7 +7,7 @@ django-tables2==2.0.3
 django-taggit==0.23.0
 django-taggit-serializer==0.1.7
 django-timezone-field==3.0
-djangorestframework==3.9.0
+djangorestframework==3.9.1
 drf-yasg[validation]==1.14.0
 graphviz==0.10.1
 Jinja2==2.10