setup app for heroku

- https://devcenter.heroku.com/articles/deploying-spring-boot-apps-to-heroku#customizing-the-boot-command
- https://blog.codecentric.de/2015/10/spring-boot-anwendungen-bei-heroku-deployen/
- https://github.com/britter/spring-boot-heroku-demo
- https://exampledriven.wordpress.com/2016/11/04/spring-boot-heroku-example/