Use java 1.8 for travis.yaml
diff --git a/.travis.yml b/.travis.yml
index 52197b3..124c836 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -11,3 +11,5 @@
 #  limitations under the License.
 
 language: java
+jdk:
+  - oraclejdk8