Return array of version uris

Fixes #625 and #627. Returns array of uris for every version. This also addresses issues with fixity checking in Sufia where you can now check the fixity of a specific version:
```
object.versions.each do |uri|
  ActiveFedora::FixityService.new(uri).check
end
```
how about `version_uri` because we are returning the URI, not the version metadata or content.