Looking for wrong version

Beside this [issue](https://github.com/RiotGames/berkshelf/issues/906), I'm running into an issue where Berkshelf doesn't constrain itself to the available versions.

To reproduce:

	git clone git@github.com:opscode-cookbooks/nginx.git
	git checkout v2.0.4

Run berks and it'll blow up trying to grab `yum 2.4.3`:

	berks                                                                                                                                                                  
			
	DEPRECATED: Your Berksfile contains a site location pointing to the Opscode Community Site (site :opscode). Site locations have been replaced by the source location. Change this to: 'source "http://api.berkshelf.com" to remove this warning. For more information visit https://github.com/RiotGames/berkshelf/wiki/deprecated-locations
			
	building universe...
	Using nginx (2.0.4) path: '/private/tmp/nginx'
	Using apt (2.3.0)
	Using bluepill (2.3.0)
	Using build-essential (1.4.2)
	Using ohai (1.1.12)
	Using runit (1.4.0)
			
	Installing yum (2.4.3)
	yum (2.4.3) not found in any sources



A quick look at yum reveals the [highest available is 2.4.2](http://cookbooks.opscode.com/api/v1/cookbooks/yum).

Any ideas?

PS: using Berkshelf (3.0.0.beta3) or master build, and also switched the deprecated location.