Minor doc update

pointing at pombola/south_africa docs which will tell more
(currently in issues/1228-document-hansard branch)
Stop checking new Hansard sources before a historical limit

Defaults to 2009-04-22 as per Geoff's suggestion, but can be reset with
the new --historical-limit yyyy-mm-dd flag.

Fixes https://github.com/mysociety/pombola/issues/1218
Pin to sayit's pombola branch
Revert "Require django-sluggable."

We don't need django-sluggable while we're pointing at sayit's
pombola branch

This reverts commit a1e4eb29b41a1f433df1de8b954cb0d7e22985a5.