Bug 20854: Allow correct redirect on logout for Cas servers 3.0 and superior.

Bug 20854: Allow correct redirect on logout for Cas servers 3.0 and superior.

Test plan:

 1) Apply the patch
 2) Set the system preference casLogout to "Yes"
 3) Set the new system preference CasServerVersion to "CAS 3 or superior"
 4) Check that you are redirected to Koha after a CAS logout from a CAS 3 server
 5) Set the new system preference CasServerVersion to "CAS 2 or inferior"
 6) Check that you are redirected to Koha after a CAS logout from a CAS 2 server

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>