Replace examples using the outdated Django-1.4.2 with the latest version, Django-1.5.1.

Django-1.5.1 is the latest version, so it makes sense that the examples for how to install Django should use 1.5.1 instead of the outdated 1.4.2. Also, Django no longer supports Python 2.5, so I've updated the book to noted support for 2.6.5-2.7.