Hm, looks like the “headless” download is no longer supported by Oracle. When triggering a download from their web site (https://www.oracle.com/technetwork/java/javase/downloads/jdk11-downloads-5066655.html) you’re forwarded to the Oracle login page.
Well, will wait until the end of this week – but install-jdk.sh's support of the Oracle JDK is about to end. Perhaps Travis should forward from oraclejdk* to openjdk* automatically?