.travis.yml: install open-jdk6, as it was removed from the default image

PascalSchumacher · PascalSchumacher · commit 9bdb3dbe184a · 2017-09-21T21:06:48.000+02:00
(see travis-ci/travis-ci#8199)
diff --git a/.travis.yml b/.travis.yml
@@ -16,6 +16,11 @@
 language: java
 sudo: false
 
+addons:
+  apt:
+    packages:
+      - openjdk-6-jdk
+
 jdk:
   - openjdk6
   - openjdk7
