(maint) Remove natty from build_defaults, changelog

Ubuntu's Natty has been EOL for 3 months
[http://en.wikipedia.org/wiki/List_of_Ubuntu_releases#Ubuntu_11.04_.28Natty_Narwhal.29],
so this commit removes the natty cow from build_defaults and also removes natty
from the changelog template. It also adds quantal, which had been previously
neglected, to the changelog template.