Migrate to Java 11

Java 11 brings a number of benefits, but at the moment, tooling support for anything over Java 8 is a bit patchy, so I'm maintaining an open branch which does the migration so we can flip when it makes sense to do so.

GraalVM is a more capable replacement for Nashorn, which is deprecated, but again isn't ready (no v1.0.0 yet). #229 will follow up on that.