Update SpringSecurityKerberosGrailsPlugin.groovy

change the package import corresponding to package naming changes in spring-security-core 2
http://grails-plugins.github.io/grails-spring-security-core/docs/manual/guide/newInV2.html
Merge pull request #2 from MatrixCrawler/patch-1

Update SpringSecurityKerberosGrailsPlugin.groovy