[CI] Remove activator dependency

The current Travis CI tests depend on Lightbend `activator` builds that have been retired by the play framework in [2017](https://www.lightbend.com/blog/introducing-a-new-way-to-get-started-with-lightbend-technologies-and-saying-goodbye-to-activator). Its probably a good idea to switch to pure `sbt` builds.

Start by inspecting the Travis CI configuration and the commands that invoke Lightbend activator. Replace the retired activator build path with pure sbt commands, then run the CI-equivalent build locally if available. Done means the Travis CI build completes without depending on activator.

Written by the indexing model from the issue text.