Add an heroku command to accommodate to new heroku stack setting

Author: Xiaohong-Deng

getting_set_up.md

@@ -39,6 +39,7 @@ Then from within your copy of the Typo repository, run:
 
 ```
 $ heroku create
+$ heroku app:stacks:set cedar-14
 $ git push heroku master
 $ heroku run rake db:migrate
 $ heroku open # doesn't work on C9 --> on C9 right click on link that git push to heroku generates