Modernize the builder

* Uses the buildpack installer for custom buildpacks
* Uses helpers to format the output
* Uses the environment variables for the start & exec commands
Remove old buildchain
Update Authors, License and add changelog
Remove preinstalled custom buildpacks
Merge branch 'feature/modernize' into develop

* feature/modernize:
  Remove preinstalled custom buildpacks
  Update Authors, License and add changelog
  Remove old buildchain
  Modernize the builder
  Use flynn buildpack installer
  Use Heroku Stack Images as baseimage
  Use debootstrap ubuntu flavor