Replace Travis CI with GitHub Actions

[jakzal]

• [x] Build & test [pass]
• [x] Cache maven deps
• [x] Notify slack
• [x] Remove legacy configurations
• [x] Configure repository secrets
  • SLACK_WEBHOOK

[jakzal]

For some reason, vlingo-schemata isn't published to bintray.

Also, there's a circleci configuration in place, which I'm not sure if is still used/needed.

[jakzal]

I'm going to merge the Travis CI migration now and consider replacing/reimplementing CircleCI in a separate PR.