Feature: Adding ppa support for apt_repository

This PR adds ppa url support to apt_repository.

Example:
```
apt_repository "php55" do
  uri 'ppa:easybib/php55'
  distribution node["lsb"]["codename"]
end
```

I am not sure about the correct style here: Rubocop fails using the `.rubocop.yml`, but pretty much everywhere. Since the last Travis-run is 6 month ago, I guess this is just not up to date or used. If I should change/fix sth, feel free to ping me.

fixes https://github.com/opscode-cookbooks/apt/issues/89