Update list of missing Scala 2.12 dependencies

[adamvoss]

akka-http-core and ammonite-repl both seem to have Scala 2.12 compatible builds. However, Play support for 2.12 is coming in the yet unreleased 2.6 (playframework/playframework#6691) so the play-ws dependency has no available target at the moment.

This updates the build.sbt to reflect these facts.

[wsargent]

Please see https://github.com/playframework/play-ws which does have a 2.12 target and a milestone build. There's still some hairiness in the shaded dependencies, but you can use https://github.com/wsargent/play-ws-cli as an example.