ubuntu/devel points to an old series instead of the current one

In https://code.launchpad.net/~usd-import-team/ubuntu/+source/bind9/+git/bind9, ubuntu/devel points to Xenial and not Bionic.

I think this is because we're resetting ubuntu/devel in _update_devel_branches unconditionally. I remember being puzzled by this before, but I think I forgot about it later. I'm not sure how it previously worked (or if it did).

Perhaps we should check if the series being imported is the active series? Or otherwise, what should the algorithm be?

We should also consider what incorrect branch pointers need fixing up, or if it's worth it given that we'll be re-importing the world at some point anyway.