Updated `omnibus` with `*.metadata.json` fixes

The latest version of Omnibus contains important fixes for the generated metadata files. In particular the following items are fixed:

* `*.metadata.json` files are properly copied over to $PWD/pkg at build completion on Windows builders: opscode/omnibus-ruby@f90842b
* Latest version of Ohai properly retrieves platform and platform version on Windows builders: opscode/omnibus-ruby@bfedf87
* Build iteration has been added to `*.metadata.json` for use in publishing tasks: opscode/omnibus-ruby@3dfd9b3

This PR also partially addresses issues raised in opscode/oc_post_mortems#33.

/cc @opscode/release-engineers @opscode/client-engineers