Create default matrix exclusions

[rossabaker]

http4s builds all the Scalas on the base Java, and all the Javas on one preferred Scala. This might be sensible throughout the org.

[armanbilge]

We've since adopted this pattern in the CE build too. I wonder if it can be pushed up an sbt-spiewak default.

[rossabaker]

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.