Fix extension support when there is no settings.SITE_ROOT.

This was a regression introduced in this release. settings.SITE_ROOT is
not a requirement in the extensions code, and isn't available in tests,
so the code is now conditional on its existence.
Require Django 1.4.5 as a minimum version.

Django 1.4.5 is the latest security release. It's now our minimum
version. We also are, for now, disabling compatibility with 1.5, until
we can be sure it works.
Release Djblets 0.6.11.