Add the heroku-buildpack-apt buildpack to your project:
heroku buildpacks:add --index 1 https://github.com/heroku/heroku-buildpack-apt.git
Add this buildpack to your project:
heroku buildpacks:add --index 2 https://github.com/josduj/heroku-buildpack-unoconv.git
Create an Aptfile file in your project root with the following content only:
unoconv
Redeploy your project after committing the above file.