Fix excluding schema view from doc for DRF 3.9

Author: Aleksei Mikhailov

rest_framework_swagger/views.py

@@ -15,6 +15,7 @@ def get_swagger_view(title=None, url=None, patterns=None, urlconf=None):
     class SwaggerSchemaView(APIView):
         _ignore_model_permissions = True
         exclude_from_schema = True
+        schema = None
         permission_classes = [AllowAny]
         renderer_classes = [
             CoreJSONRenderer,