Sorry for the troubles.
Since you aren’t specifying a dist: in your .travis.yml file, this is a side-effect of our migration to change the default distribution from Trusty to Xenial.
In brief, your repository’s default changed and oraclejdk9 isn’t supported on Xenial.
The workaround is to either add dist: trusty to your .travis.yml file or use openjdk9 .
Hope this helps!