Configure Travis to build on JDK8 and JDK11

.travis.yml

@@ -1,7 +1,8 @@
 language: java
-dist: trusty   # Xenial build environment won't allow installation of Java 8
+dist: xenial
 jdk:
-- oraclejdk8
+- openjdk8
+- openjdk11
 
 env:
   global:
@@ -21,12 +22,6 @@ after_success:
 - mvn clean org.jacoco:jacoco-maven-plugin:prepare-agent package sonar:sonar -Dsonar.host.url=https://sonarcloud.io -Dsonar.login=$SONAR_TOKEN
 - bash update-ghpages.sh
 
-# use latest java version available instead of travis default
-addons:
-  apt:
-    packages:
-      - oracle-java8-installer
-
 notifications:
   email:
     - iluwatar@gmail.com