Don't update apt sources pointing to mirrors.
Don't update apt sources pointing to mirrors.

This change stops babushka from writing a new apt source file (and
running apt-get update) on ubuntu platforms where the apt source points
to a country mirror.

Example: I tend to configure apt sources to point to local (au) mirrors
before installing babushka on dev vms. Also recent vagrant boxes come
preconfigured with apt sources that look like this:

  deb http://us.archive.ubuntu.com/ubuntu natty main restricted

The "xx.archive" format unfortunately makes the met block in the
'apt source' dep fail.

Babushka should now recognise dep sources in the following formats:
http://archive.ubuntu.com/ubuntu
http://au.archive.ubuntu.com/ubuntu

For completeness I've checked that the following formats don't match:
http://wwwarchive.ubuntu.com/ubuntu
http://www.archive.ubuntu.com/ubuntu
http://a.archive.ubuntu.com/ubuntu
http://.archive.ubuntu.com/ubuntu

victorklos/babushka,victorklos/babushka,victorklos/babushka