Prevent building under Java 9

Prevent building under Java 9

via maven-enforcer-plugin requireJavaVersion [1.8.0,1.9.0)

beacause I've just supported the first user running into
weird issues when (unknowingly, I'm sure) running ODL
build under Java 9; see:
https://stackoverflow.com/questions/47661667/failed-to-build-the-sal-distributed-datastore-error-while-generating-javadoc-in

I've locally tested that this works and fails under Java 9.

Bleeding edge kernel developers can still
work on Java 9 via -Denforcer.skip=true, of course.

Change-Id: I889958d88b2f6b4d28fe816454fb165771cb92a7
Signed-off-by: Michael Vorburger <vorburger@redhat.com>