maven compiler plugin config and fixed ambiguous function in test/JenkinsRule

setting the compiler config params to 1.6 makes m2e happy, because the code contains Override annotations on functions that implement an interface which the 1.5 compiler doesn´t like

I got an ambiguous function error in test/JenkinsRule. The underlyig code seems to do the same thing

The other commits I fetched and rebased from jenkinsci:master