made more robust. See...

made more robust. See http://www.nabble.com/How-to-reference-library-JARs-of-a-plugin-at-runtime-%28URL%29-td15601148.html#a15601148


git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@7619 71c3de6d-444a-0410-be80-ed276b4c234a