Update werkzeug import

- `cached_property` was deprecated in Werkzeug>0.16.1
- a replacement is available in `werkzeug.utils`

Example issue:
noirbizarre/flask-restplus#777

flaskext/genshi.py

@@ -18,7 +18,7 @@
 
 from genshi.template import (NewTextTemplate, MarkupTemplate,
                              loader, TemplateLoader)
-from werkzeug import cached_property
+from werkzeug.utils import cached_property
 from flask import current_app
 
 try: