Change effective on 03 December 2018
We have updated the Cedar-14, Heroku-16, and Heroku-18 stack images. This new version addresses security issues on several packages, and installs gcc on the Heroku-16 and Heroku-18 stacks (it was previously only available at build time).
The new stack images 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, Heroku-16, and Heroku-18 stack images.