Travis: Build and test on JDK 7, 8 and 11.

This uses the Trusty (14.04) image since that in the only one
where all three JDKs are available.

.travis.yml

@@ -1,2 +1,8 @@
+dist: trusty
+
 language: java
 
+jdk:
+  - openjdk7
+  - openjdk8
+  - oraclejdk11