Officially require Python 2.6+

We have no way to test Python 2.4 compatibility anymore, and that
version is long deprecated. From version 2.8+ require Python 2.6 or
above (but still not 3.0).

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>