Start testing on java 9

Author: Bryan Donlan

.travis.yml

@@ -2,6 +2,8 @@ language: java
 jdk:
   - openjdk8
   - oraclejdk8
+#  - openjdk9 # Not yet supported by travis
+  - oraclejdk9
 script: mvn install -Dgpg.skip=true
 sudo: false
 dist: trusty