Release jar artifacts to bintray for plugin development

[frsyuki]

Plugins need to depend on io.digdag:digdag-spi package. But now jars are released to a maven repository built on a private S3 bucket. It's not publicly accessible. Idea here is to move the repository to bintray. Actions:

• [x] Create new organization on bintray
  • https://bintray.com/digdag
• [x] Invite core developers to the organization
• [ ] Update digdag download proxy server (dl.digdag.io) to support bintray. See embulk dl proxy: https://github.com/embulk/plugin-crawler
• [x] Optional: Move all old releases to bintray
• [ ] Update build.gradle to release files to bintray.

[frsyuki]

download proxy changes: https://github.com/treasure-data/digdag-dl-proxy/pull/1