(maint) Remove natty from build_defaults

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.
Merge pull request #415 from haus/remove_natty_from_cows_list

* Remove natty from build_defaults

Since the distribution no longer exists upstream, stop building packages for it.  Users who were on the platform are advised to upgrade to a supported version of Ubuntu.