Make sure prey.sh is only ever run as root

Earlier today I ran `./prey.sh` not as root. This messed up things as it installed a cron for my user (not root) which subsequently failed (noisily) every time it was run (since writing to /var/log/prey.log was failing with permission denied)

It's also quite misleading being able to run `./prey.sh --check`
It seemingly works fie, but confusingly notifies you that the cron is missing and you should install it (because it's actually set for root, not the current user)