We don't technically need Elasticsearch to align the Lucene version with our version, but being able to run the integration tests optionally on Java9 is a nice step forward.

Java 9 doesn't allow access to the buffer cleanup methods used by the MMapDirectory; this version of Lucene correctly detects the problem and automatically switches to a lesser performing Directory implementation. The MMapDirectory works fine with Java9 since Lucene 6+ again.