Travis: install the openjfx package

This is an attempt to keep the code building with OpenJDK 8.
If this fails, we can migrate to building with OpenJDK 11,
and targeting Java 8, although that is less good in some ways.

The openjfx package is not available on trusty, so we use xenial.

Author: ctrueden

.travis.yml

@@ -1,9 +1,14 @@
+dist: xenial
 language: java
 jdk: openjdk8
 branches:
   only:
   - master
   - "/.*-[0-9]+\\..*/"
+before_install:
+  - sudo apt-get update
+  - sudo apt-get install -y openjfx=8u161-b12-1ubuntu2 libopenjfx-java=8u161-b12-1ubuntu2 libopenjfx-jni=8u161-b12-1ubuntu2
+  - sudo apt-mark hold openjfx libopenjfx-java libopenjfx-jni
 install: true
 script: ".travis/build.sh"
 cache: