[packaging] Remove natty from the deb pkg list, its EOL

Ubuntu Natty is EOL, and we are no longer going to build or ship for this
distribution. This commit removes it from the default list of distributions we
build for in facter.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #398 from MosesMendoza/maint/1.6.x/remove_natty_from_cow_list

[packaging] Remove natty from the deb pkg list, its EOL