Added none to vtpversion check

This resolves an issue with Cisco switches where they return none for the vtpversion even when vtp is running.

http://gorthx.wordpress.com/2011/12/09/snmp-snarky-network-management-protocol/

reported by gadago on irc, he's also confirmed this fix resolves the issue.

It's not a bug as confirmed by cisco's bug tool and I've put this fix on our install with 80+ devices and vlans discovery has gone through with no issue.