update grails.plugins.springsecurity to grails.plugin.springsecurity htt...

Update config to get rid of warning about new namespace for spring security
springsecurity.ReflectionUtils Your security configuration settings use the old prefix 'grails.plugins.springsecurity' but must now use 'grails.plugin.springsecurity'

http://grails-plugins.github.io/grails-spring-security-core/docs/manual/guide/newInV2.html