Fix Travis-CI config; test against more JDK versions

Change-Id: Ia1ab497db05788da193e5ac86796f5df68ac8533
diff --git a/.travis.yml b/.travis.yml
index e358d25..505db7d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,8 @@
 language: java
 jdk:
-  - oraclejdk7
+  - oraclejdk8
+  - oraclejdk9
+  - oraclejdk10
+  - openjdk8
+  - openjdk10
+  - openjdk11