Clarify Python/Django requirements

Nothing really prevents Django 1.3 from working but the tests won't run with it -- it doesn't have `override_settings()`.

Some code already prevents Graphite-web from running on Python 2.5. Travis doesn't even support it so it's hopeless to claim 2.5 support.

This patch syncs requirements with what we actually test on Travis, which is the sanest approach.

The INSTALL file also keeps getting out of sync with what's in `docs/`. Let's keep things up-to-date in one place.
Uh yeah I meant 2.6. Fixing :)