older versions of zfs command don't support upgrade and spew errors out of facter

On some old version of the zfs command the upgrade subcommand is not included. This results in the standard error getting thrown across the terminal when using facter. resolution.rb just seems to use:
out = %x{#{code}}.chomp 
So redirect errors to /dev/null in the fact itself seems like a suitable fix in this case.

# zfs upgrade -v 2>/dev/null
# sysadmin@fnet-flash1 ] $ echo $?
2


# facter
unrecognized command 'upgrade'
usage: zfs command args ...
where 'command' is one of the following:
<snip>