Fix to work with Ubuntu 14.04

Docker updated the ubuntu dockerfile to use 14.04 (see: https://github.com/dockerfile/ubuntu/commit/c07fc04d3aa96d454766b25f2d354f5d977218be)

In 14.04 `python-software-properties` was renamed: `software-properties-common` (see: http://stackoverflow.com/a/16032073)