Default matrix exclusions

[rossabaker]

Several projects are building all the Scalas for one Java, and all the Javas for one Scala, instead of the cartesian product. Would this be a sensible default to keep build matrices under control?

http4s/sbt-http4s-org#110

[armanbilge]

Continuing from http4s/sbt-http4s-org#110 (comment).

That sounds good to me, but we don't know the default Scala or Java there the way that we do within the org. Could pick a head or tail, but that assumes everyone sorts them the same way.

I think the head/tail already have significance though: they determine the default Scala when you start sbt and also the JDK used for the publish step. So those seem like reasonable choices.