bump to 3.1.6 for sage 6.1 (potentially) restrict other version to <ntl-6.0.0

Package-Manager: portage-2.2.7
singular 3.1.6 in at last. older version are not patched to use ntl-6 so restrict dep for 3.1.5.

Package-Manager: portage-2.2.7
Mask (lib)singular 3.1.6 to use for sage 6.1 (cross fingers). singular 4 also got released but it is truly a major revision.