Publish GitHub release artifacts

[labianchin]

It would be good to provide GitHub release artifacts from sbt-pack task running on travis.

There is some good example of this in ratatool: https://github.com/spotify/ratatool/blob/master/.travis.yml#L27

[rulle-io]

@labianchin Please, provide more context here.
Since ratatool moved from Travis CI to Circle CI, do you still think the .travis.yml is a good example?
Working link to .travis.yml