Pushing documentation for new CentOS installation method.

The new documentation resolves issues GH-575 and GH-576:

1. The Vagrant installation has been verified to work with VirtualBox 4.3.8, so the warning about not using VirtualBox 4.3.* or higher has been removed.

2. The Python SCL installation method, which now installs Python 2.7.5 has been replaced by an altinstall installation procedure that keeps the system on Python 2.7.3.