Update Configuring-LongTerm-Authentication.md

It looks like org.jasig.cas.authentication.principal.RememberMeUsernamePasswordCredential has been moved to org.jasig.cas.authentication package.

Related discussion.
Remember me with 4.0.0
https://groups.google.com/forum/#!topic/jasig-cas-user/jpO8qFnX_BI

Remember ME unable to load class #636
https://github.com/Jasig/cas/issues/636