Require Minimum Plugin Version

This adds a method to JobManagement that logs a warning and sets the build status to unstable if the installed version of a plugin is older than the required version.

It's used to require version 4.0 or later of the Warnings Plugin. See https://groups.google.com/forum/?fromgroups#!topic/job-dsl-plugin/NipfXXfHyE0 for details.