Add Migration Guide Snippets for Cloud Tasks [(#2316)](GoogleCloudPlatform/python-docs-samples#2316)

* Migration guide

* remove app and update migraitonn

* snippets for migration guide - tests added

* lint

* remove print statements

* Styling changes

* Travis trigger

Author: averikitsch

samples/snippets/requirements.txt

@@ -1,4 +1,2 @@
-Flask==1.0.2
-gunicorn==19.9.0
 google-cloud-tasks==1.1.0
 googleapis-common-protos==1.6.0