json metadata should be preferred over ruby metadata

In all cases the compiled metadata should be preferred by clients over the Ruby representation. The current behaviour of the Chef-Client reads the metadata.rb first (and that's why I originally wrote it this way), but I discussed this with @danielsdeleo and he agreed that it is not the correct behaviour.

See:
* https://tickets.opscode.com/browse/CHEF-4811
* https://tickets.opscode.com/browse/CHEF-4810

/cc @ivey @sethvargo @KAllan357