Travis CI with Heroku CLI

.travis.yml

# use containers in Travis (instead of VMs)
sudo: false

# install Heroku CLI 
# (based on https://github.com/travis-ci/apt-package-whitelist/issues/375)
addons:
  apt:
    sources:
      - heroku
    packages:
      - heroku-toolbelt
      
# install additional Heroku CLI plug-ins
before_script:
  - heroku plugins:install <package-name>
  
# more docs here:
# https://docs.travis-ci.com/user/migrating-from-legacy/#How-Do-I-Install-Custom-Software%3F