Missing deps

Implements feature to be used in builder as discussed
version_for and version_of are returning the most relevant version?  What does that mean?  The version per system with the most builds?  I guess I don't understand what the requirement comes from or what we're trying to solve here.

The examples in your tests demonstrate it but I don't follow why:
```ruby
        versions = {:koji => ['1.0', '2.0', '1.0'],
                    :git  => ['3.0', '3.0'],
                    :yum  => ['2.0', '1.1', '2.0']}
        described_class.should_receive(:versions_for).and_return(versions)
        expected = {:koji => '1.0',
                    :git  => '3.0',
                    :yum  => '2.0'}