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.
Remove uses of sys.exc_info in favor of the py2.6+ syntax
Stop referencing INSTALL
2.6 or greater
Merge branch 'master' into fix/versions
Merge pull request #794 from brutasse/fix/versions

Clarify Python/Django requirements