Fix #1206: Update to sbt 0.13.6

Also fix the sbt version for the sbt-plugin-test build.
Merge pull request #1213 from gzm0/sbt-0.13.6

Fix #1206: Update to sbt 0.13.6
Fix #1208: toString for Semantics and CheckedBehaviors

Looks like this:

    Semantics(
      CheckedBehaviors(
        asInstanceOfs = Unchecked
      ),
      strictFloats = false
    )
Fix #1212: Run CI against Java 1.{6-8}
Merge pull request #1220 from gzm0/sem-tostring

Fix #1208: toString for Semantics and CheckedBehaviors
Buglist partests arrayclone-{old,new} and issue192.

They're broken on master.
Merge pull request #1221 from gzm0/cross-jdk

Fix #1212: Run CI against Java 1.{6-8}
Merge pull request #1223 from sjrd/buglist-broken-partests

Buglist partests arrayclone-{old,new} and issue192.