Now works on distros without 'dpkg'.

Hi Mark,

Just listened to UUPC and heard about this project and decided to package it for Arch Linux.

Although the `subprocess` call to `dpkg` is inside a `try:` it causes a `raise child_exception` on distros where `dpkg` is not available. This pull-request addresses that and I can now run your indicator on Arch Linux under MATE 1.6 :-)

Regards, Martin.