Update django-polymorphic from 2.0.3 to 2.1.2 (#677)

Author: pyup-bot

requirements-development.txt

@@ -1,7 +1,7 @@
 -e .
 django-debug-toolbar==1.11
 django-filter==2.1.0
-django-polymorphic==2.0.3
+django-polymorphic==2.1.2
 Faker==2.0.0
 factory-boy==2.12.0
 flake8==3.7.7