UPDATE: as pointed out in the comments this is no more needed because since 2018 heroku changed its default behaviour and dev dependencies are automatically installed
ORIGINAL ANSWER
Heroku by default installs only the production dependencies, ignoring the development dependencies under devDependencies.
Setting the npm production variable to false do the trick:
heroku config:set NPM_CONFIG_PRODUCTION=false
More info are available at the Heroku Node.js Support page.