Vendor `chef-gem` and `libffi` software definitions

Ohai 7.2.0 was just released and has a dependency on the `ffi` gem which ships with a native extension which requires `libffi`. The fix is to add a proper dependency on `libffi` in the `chef-gem` software definition.

We can’t simply update to the latest ominibus-software as this project is still on Omnibus 2.0.x. The alternate is to vendor the affected software definitions tweaked for the Omnibus 2.0 DSL.

This has been successfully tested in CI also:
http://wilson.ci.opscode.us/job/chef-server-trigger-ad_hoc/14/downstreambuildview/

/cc @opscode/release-engineers @opscode/server-team @mmzyk