Scala 2.12 has better support for newer Java versions and includes additional
compiler warnings that are helpful during development. In addition, Scala 2.11
hasn't been supported by the Scala community for a long time, the soon to be
released Spark 2.4.0 will finally support Scala 2.12 (this was the main reason
preventing many from upgrading to Scala 2.12) and Scala 2.13 is at the RC stage.
It's time to start recommending the Scala 2.12 build as we prepare support for
Scala 2.13 and start thinking about removing support for Scala 2.11.

In the meantime, Jenkins will continue to build all supported Scala versions (including
Scala 2.11) so the PR and trunk jobs will fail if people accidentally use methods
introduced in Scala 2.12.

Reviewers: Ewen Cheslack-Postava <me@ewencp.org>