Fix dependency for Ubuntu > 12.04

Appearently `python-software-properties` has been renamed to `software-properties-common` in Ubuntu. So unless support for Ubuntu 10.04 is a requirement for you I'd recommend switching to `software-properties-common`.
See 
http://packages.ubuntu.com/search?keywords=python-software-properties
and
http://packages.ubuntu.com/search?keywords=software-properties-common
for comparison.

Cheers,
tablet-mode