debian/control: Replaces supybot

It was just said at #Limnoria that someone couldn't install Debian packages
 of Limnoria, because haven't been configured to replace supybot.

NOTE: I am not Debian packager, so I have no idea if this works, so please
test it first!
Merge pull request #620 from Mkaysi/debian

debian/control: Replaces supybot