SI-8966 Allow use of jvm-1.8 via the Ant scalac task

This option has been allowed by the command line compiler since
ee706b873a28.

This commit allows use of this via Ant.

Note: we still don't exploit the features of classfile version 52,
but watch this space as we roll out method handle based closures
soon!
Merge pull request #4104 from retronym/ticket/8966

SI-8966 Allow use of jvm-1.8 via the Ant scalac task