Fixes for minor crash bugs and Ubuntu compatibility

Hello,

Here are some changes that I've had to make in order to deploy and upgrade an Ubuntu system with Dust 0.16.2.

There are two small changes to avoid crashes, and I've also switched the package management utility. Ubuntu use apt-get but no longer ship Aptitude, and have a separate utility that they recommend for upgrading distribution versions ("update-manager"). It looks like apt-get is also slightly better than Aptitude for Debian upgrades, so I've removed aptitude altogether: http://raphaelhertzog.com/2011/06/20/apt-get-aptitude-%E2%80%A6-pick-the-right-debian-package-manager-for-you/ 

Dust has been really fast and easy to work with so far - thank you for releasing it!