Set zfs-auto-snapshot to use UTC time for snapshots

I ran into a bug when deploying this + Samba4 for Previous Versions to a Windows 7 client - all of the snapshots were appearing as 'Tomorrow'. It seems that Windows 7, at least, expects the date given to be in UTC. As I'm in UTC+8, this made all of the dates 8h ahead!

Just added the --utc flag to the date code there, and Windows 7 now correctly gives the date. This shouldn't affect anything else, since cron takes care of scheduling. UTC is also better idea to be storing the snapshot date in, as well.