Use flynn buildpack installer

The flynn buildpack installer allows installing a specific version of a buildpack giving more control over the final buildstep environment.

See https://github.com/flynn/flynn/blob/master/slugbuilder/builder/install-buildpack for the upstream source
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