Update Cloud Tasks library version [(#2516)](GoogleCloudPlatform/python-docs-samples#2516)

* Update to new library

* update library version

Author: averikitsch

samples/appengine/flexible/tasks/requirements.txt

@@ -1,4 +1,4 @@
 Flask==1.1.1
 gunicorn==19.9.0
-google-cloud-tasks==1.2.1
+google-cloud-tasks==1.3.0
 googleapis-common-protos==1.6.0