Build with OpenJDK 6 on Travis CI

The artifacts that are published to Maven Central are built with
OpenJDK 6, too.

.travis.yml

@@ -7,10 +7,13 @@ install:
   - ./mvnw test -DskipTests
   # Delete all files created with JDK 8
   - ./mvnw clean
-  # Restore desired JDK
-  - export JAVA_HOME=$ORIGINAL_JAVA_HOME
+  # Build with JDK 6 because we build the artifacts that are published
+  # to Maven Central with OpenJDK 6, too.
+  - jdk_switcher use openjdk6
   # Run original install command but without GPG signing
   - ./mvnw install -DskipTests=true -Dgpg.skip=true
+  # Restore desired JDK
+  - export JAVA_HOME=$ORIGINAL_JAVA_HOME
 addons:
   apt:
     packages:

README.md

@@ -53,7 +53,8 @@ The basic coding style is described in the
 System Rules supports [Travis CI](https://travis-ci.org/) (Linux) and
 [AppVeyor](http://www.appveyor.com/) (Windows) for continuous
 integration. Your pull request will be automatically build by both CI
-servers.
+servers. On Travis CI we build your pull request with OpenJDK 6 and run test
+with different JDKs (Java 6 to 9).
 
 
 ## Release Guide