Verify against JDK9 and Spring5

[ultraq]

Both Java 9 and Spring 5 came out recently, so it might be good to verify that everything here still works, plus to see if there's anything in the new versions that I can work towards in to keep things up-to-date.

[ultraq]

After adding Java 9 to the CI build, it started failing because of the older version of Groovy. Had to up that to 2.4.13 (I think people reported that at least 2.4.12 was needed?) to get compilation and tests working again. So that's Java 9 sorted. Need to still try a Spring 5 (or Spring Boot 2?) project to make sure there are no surprises there either.

[ultraq]

Cool, works using Spring Boot 2.0 RC1 and Java 9 👍