[JENKINS-18403] Commented-out bytecode level restriction for Maven process.
As in StreamTaskListener, the returned PrintWriter must be set to autoflush.
Otherwise the idiom t.printStackTrace(listener.error("…")) does not work.
[FIXED JENKINS-18403] Detect when an attempt is made to run Maven on JDK 5.
If so, retry with (6+) slave JVM, setting maven-{compiler,surefire}-plugin properties to point to original.
Merge branch 'master' of github.com:jenkinsci/jenkins