Fix building with OpenJDK 6 in Travis CI

OpenJDK 6 is no longer supported in the default build environment of
Travis CI, so downgrade to Ubuntu 14.04 LTS (Trusty Tahr). This version
is EOL (like Java 6), but seems to be the only way to continue building
with OpenJDK 6. Even with the downgraded build environment, a manual
workaround is necessary since the usual "jdk" key does not work.

.travis.yml

@@ -1,3 +1,5 @@
+# openjdk-6-jdk is not available in more recent Ubuntu versions
+dist: trusty
 language: java
 install:
   # Download dependencies with JDK 8 because Maven Central supports
@@ -23,4 +25,10 @@ jdk:
   - oraclejdk9
   - oraclejdk8
   - openjdk7
-  - openjdk6
+
+# The jdk key does not work for openjdk6 in Travis CI any more, so
+# we manually install openjdk-6-jdk and set JAVA_HOME appropriately
+# (https://github.com/travis-ci/travis-ci/issues/9713)
+matrix:
+  include:
+  - env: JAVA_HOME=/usr/lib/jvm/java-6-openjdk-amd64