Hi @benjamin-rentlytics! Are you using CircleCI 1.0? If so, which image are you currently using (12.04 or 14.04)?
Projects having trouble with Heroku lately are usually using the Ubuntu 12.04 (Precise) build image (this has been 100% of the Heroku CLI issues I’ve seen recently). 12.04 is End of Life, so going forward, we’ll only be modifying the image to add critical bug fixes (not new features or new versions of existing tools).
We’ve got some information to help you migrate to 14.04 (Trusty).
You can change the build image via the menu in Project Settings (gear icon) > Build environment.
Once that’s done, you’ll need to trigger a new build by pushing a commit (just running a rebuild will reuse the same build image, which is what we want to avoid).