Test sbt 1.0.x on JDK 9

[dwijnand]

[retronym]

Referencing #2951, which adds some basic compatibility with Java 9 to the 0.13 branch.

[dwijnand]

scala/scala-dev#139 is the Scala ticket about support JDK 9 in general.

Also @smarter reports on sbt/zinc-contrib that:

Guillaume Martres @smarter Apr 25 15:43
Ah, the good news is that this seems to be fixed in zinc: I can reproduce the problem in sbt 0.13.15 but not 1.0.0-M5