Fix incorrect usage of phpCAS on PT renewal

Using retrievePT without updating the phpCAS client from session info
will result in an incorrect call to the CAS server :

Error while retrieving new PT: PT retrieving failed
(code=`INVALID_REQUEST', message=`'pgt' and 'targetService' parameters are both
required')

This only happens in cases where the PT becomes invalid without
expiring, for instance if the credential cache does not manage multiple
credentials, or if it expires for any reason other than timing out.