Bump `rbenv` to 1.6.5; Add a workaround for [CHEF-3940]

There is too much pain on that ticket to summarize here, go read:
https://tickets.opscode.com/browse/CHEF-3940

We need the a new version of the `rbenv` cookbook that actual exposes
the `node['rbenv']['user']` attribute.
Ensure `/etc/profile.d` exists on FreeBSD

rbenv drops off a file at `/etc/profile.d`, if this directory does not
exist the Chef run fails.
Update README for recent changes and all supported platforms
Ensure metadata reflects all supported platforms.