Migrate publishing off of Bintray

[SethTisue]

Because bye-bye Bintray!

However, I don't think it's necessary to do this proactively. It's quite rare that we need to publish a new dbuild version. So we can worry about this if and when the time comes.

Not sure if publishing to Sonatype exclusively is an option, because there's a tarball (.tgz) that needs to be hosted. Perhaps we could publish all the regular JARs to Sonatype, and then work out the tarball hosting with Lightbend IT, using Cloudsmith or, who knows, S3 or whatever.