Fix NoMethodError bugs in docker::aufs recipe

Fixing some bugs introduced in #35:

    NoMethodError
    -------------
    undefined method `empty?' for nil:NilClass '`

    NoMethodError
    -------------
    undefined method `Version' for Gem:Module`

Tests passing via:

1. `bundle exec kitchen test ubuntu`
2. `bundle exec foodcritic .`
3. `bundle exec rubocop`
4. `bundle exec knife cookbook test`

Note: fedora-19 currently has a problem running the docker tests, but
this was already happening before #35.  Please see [this gist][1]

[1]: https://gist.github.com/8216591