handle non-numeric NetBSD versions

I've got report from cpantesters with version '6.0.1_patch':

   # Warning was 'Argument "6.0.1_patch" isn't numeric in numeric ge (>=) at /home/smoke/.cpan/build/RedisDB-2.15-zvvBwC/blib/lib/RedisDB.pm line 287.'
bump version 2.16