Add openjdk18 build to Travis CI
Add openjdk18 build to Travis CI

apache/groovy,apache/incubator-groovy,apache/incubator-groovy,apache/groovy,paulk-asert/groovy,apache/groovy,apache/incubator-groovy,paulk-asert/groovy,paulk-asert/groovy,apache/groovy,paulk-asert/groovy,apache/incubator-groovy