flask-kitchensink: Specify werkzeug version (#200)

ProxyFix package is only available after 0.15.0 release.
https://werkzeug.palletsprojects.com/en/0.15.x/changes/#version-0-15-4

Author: tokuhirom

examples/flask-kitchensink/requirements.txt

@@ -1,3 +1,3 @@
 line-bot-sdk
 flask
-werkzeug
+werkzeug>=0.15.4