This error is because you're using Java6 and the the xar plugin version 6.0 requires java7 :)

See https://blogs.oracle.com/henrik/entry/java_7_questions_answers for example: "I have created an application in Java 7, but when my users try to run it they get an Unsupported major.minor version 51.0 error. What does this mean and what can I do about it?"

Now you could use a version < 6.0 which was still using java6, such as 5.4.4. At the time it wasn't supporting addition of license headers I think but it would at least do the formatting...