Replacing reference to deprecated PluginManagerHolder

org.codehaus.groovy.grails.plugins.PluginManagerHolder was deprecated a while ago and has been completely removed in 2.4, causing the compilation to fail with 2.4.x. I replaced it with the recommended replacement (grails.util.Holders) as per http://grails.org/doc/2.4.x/guide/upgradingFrom23.html.