Point mirror at CentOS vault

When CentOS updates it's $(VERSION) the mirrors drop the packages accessed under the previous path. *Old* versions of CentOS remain accessible at [http://vault.centos.org/](http://vault.centos.org). Making this change will allow future checkouts with this change to build the OSROLL with *old* versions of CentOS. This change still allows users to change the CentOS version for their build in `version.mk` if they choose to.