Can't install oraclejdk8 because travis changed default environment

[axblueblader]

https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment

Seems like all the builds are failing after May were due to this change.
@gondor Can you have a look at this

[sbrattla]

This should be solvable by either moving to openjdk8 or specifying dist:trusty in the travis.yml file.

https://travis-ci.community/t/install-of-oracle-jdk8-is-failing/4365/3