The oraclejdk10 target offered by travis is sometimes unstable due to
JDK download brittleness. We thus switch to the openjdk10
target. Additionally, add openjdk11 as a target.
For JDK10, the test suite did not run as the JDK8 binary installed in /usr/bin/ was used for the PRISM runs instead of the JDK10 binaries, even though the JDK10 bin directory comes first on the PATH. This is due to the python startup helpers (when running prism-auto for the tests) fiddling with the PATH variable, prepending /usr/bin/...
So, we now just set the PRISM_JAVA environment variable to the java binary on the PATH from the travis build script, which is then picked up correctly in the prism startup script.
Dave Parker