add more jdk versions

Author: wangjoshuah

.travis.yml

@@ -1,10 +1,16 @@
 language: java
 dist: trusty
 sudo: required
+addons:
+  apt:
+    packages:
+      - openjdk-6-jdk
 jdk:
+  - openjdk6
   - openjdk7
-  - oraclejdk7
+  - openjdk8
   - oraclejdk8
+  - oraclejdk9
 install: true
 script:
     - "./gradlew clean"