Change effective on 24 January 2018
The Heroku-16 stack image has been updated. This update is upgrading the default installed Ruby to fix the CVE-2017-14033 security issue.
The new stack image will be rolled out automatically in the next 24 hours. You do not need to redeploy your app or restart your dynos.
See this Dev Center article for an overview of the libraries available on the Cedar-14 and Heroku-16 stack images.