First because we don't really need the latest JDK in the Travis build: AFAIK, our build works fine even with older versions of the JDK.

Second because we will test the code again with a version of the JDK we chose explicitly when we run CI builds on Jenkins.

Third because it's a good idea to test with the JDK installed by default on Ubuntu (one more tested environment), and that comes for free.

And finally, because the JDK installer uses these #@$@!% URLs from Oracle which fail from time to time: it failed for several hours yesterday, and it also happened a few months ago. If we can avoid these transient failures, let's do.