Build with OpenJDK 8 instead of Oracle JDK

Oracle JDK is no longer available on Travis.

Author: stefanbirkner

.travis.yml

@@ -1,5 +1,5 @@
 language: java
 install: mvn -DskipTests -Dgpg.skip install
 jdk:
-    - oraclejdk8
+    - openjdk8
 script: mvn test javadoc:javadoc