Heroku CI test runs have stopped working since 8th May 2017

Resolution

Due to the recent change to the default stack image on 8th May 2017, new applications that have not specified a stack image will now use heroku-16, instead of the previous cedar-14.

This can cause issues with some buildpacks that have not been updated to work with the heroku-16 stack image. To ensure your application builds with cedar-14, you can update your app.json with:

{
  "stack": "cedar-14"
}

As app.json is only used to build applications, running applications are unaffected by this change.