fix refresh-mcolective-metadata for ruby 1.8

Reported via irc[1], the new harness for generating
facts.yaml doesn't work correctly under older rubies
where the json gem would usually be resolved via rubygems.


This mirrors the behaviour of facter and puppet 
binaries, but without the gating around bundler.

https://github.com/puppetlabs/facter/blob/master/bin/facter#L13
https://github.com/puppetlabs/puppet/blob/stable/lib/puppet/util/command_line.rb#L7

[1]