Created new CentOS 6.4 box with updated GuestAdditions.

* Minimal CentOS 6.4 install.
* Ran yum update on 10/17/2013.
* Installed latest VirtualBox GuestAdditions.
Updating the default manifest.

* Rbenv install is now forced to run before adding the Vim 7.4 path.
* Moved Vim 7.4 compilation into /opt/vim-src.
* Now installing Rails 3.2.15.
Updating README.md

* Removed references to vagrant-vbguest.  Was having issues with
  GuestAdditions.
* Updated Rails to 3.2.15.