(Unix Installation) RHEL/CentOS SCL method now installs Python 2.7.5, and only supports x86_64 architectures

Related to GH-568.

Changes the Unix installation documentation to note that installing Python 2.7.x on RHEL/CentOS 6.x from the Software Collections Library method installs **Python 2.7.5** instead of 2.7.3, and that this method does not support installations with i386 architectures. Only the x86_64 architecture is currently supported (see the directory of RPMs at http://people.redhat.com/bkabrda/python27-rhel-6/).

If the Makahiki installation must remain on 2.7.3 for compatibility reasons, a different installation process for CentOS will be needed.