Test also on openjdk8

Author: Jaroslav Tulach

.travis.yml

@@ -5,6 +5,7 @@ os:
 language: java
 
 jdk:
+    - openjdk8
     - openjdk11
 
 env:
@@ -18,6 +19,7 @@ install:
         tar -xzf graalvm.tar.gz
 
 script:
+    - java -version
     - mvn clean
     - mvn package
     - mvn exec:exec