[exim-cvs] exim-cvs mail should flow once more

I just looked into why we didn't have mail on the exim-cvs list.

It was all being rejected for the sender not being subscribed.

The list had whitelisted sender regexps for the previous two canonical
hostnames of the central Exim box, but not for the current canonical
hostname.

I've added:

    ^.*@hummus\.csx\.cam\.ac\.uk

in Mailman. Let's see if that's enough to fix it.