Add Mac OS X 10.10 (Yosemite) to the Chef platform mappings

This will enable the platform version to properly appear on:

	https://www.chef.io/download-chef-client/

If this looks good I will cherry-pick to the following stable branches:
* `chef-11-stable`
* `chef-12-stable`

/cc @opscode/release-engineers @opscode/client-engineers @jeremiahsnapp