Ubuntu 13.04 - add-apt-repository

Some installations of ubuntu 13.04 (Digital Ocean's Ubuntu 13.04 x64 Server in this case) require software-properties-common to be installed before being able to use add-apt-repository.

Not installing it will yield the following:

```
# sudo add-apt-repository ppa:dotcloud/lxc-docker
sudo: add-apt-repository: command not found
```