[packaging] remove natty distribution from builds

This commit removes Ubuntu natty from the list of distributions to build for,
as its EOL and we no longer build for it.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #180 from MosesMendoza/maint/1.2rc/remove_natty_cow

[packaging] remove natty distribution from builds