We hit a similar problem and we needed OpenJDK 11.0.3 or later (in our case due to https://bugs.openjdk.java.net/browse/JDK-8212885), and like you also noticed that Travis CI seems to be eternally stuck on 11.0.2 … 
In https://issues.apache.org/jira/browse/FINERACT-937, we gathered that this is because https://github.com/travis-ci/travis-cookbooks/blob/master/cookbooks/travis_jdk/files/install-jdk.sh#L200 is hard-coded to 11.0.2.
We managed to work around this by wiping Travis’ (bad, old, outdated) JDK, and just installing our own; see https://github.com/apache/fineract/pull/846 for exactly how we did that.
I’ve also posted a Feature Request for this on Use (latest) Java point versions from Ubuntu packages, instead of hard-coding old fix versions from java.net in install-jdk.sh