Add version information to edges of berks viz

No tests were run in the making of this patch.
Suppress `>= 0.0.0` versions in viz output
Add a test that actually builds a viz graph

On some levels this is an integration test, but it's pretty fast and
it's configured to just not run if dot is not installed, so it should be
fine for most people.
Make the graphviz png output test required

Also make it a little more strict: it now checks that a file is created,
not just that the "create an image" function doesn't blow up.
Use a map to make the viz code more semantic

The only downside to this is that it introduces a .each over a 1-element
Map, but that's not actually bad.
Visualizer style cleanup

Use parens, don't use "then".
Merge pull request #1286 from quodlibetor/add-versioned-edges

Add version information to edges of berks viz