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
```
I think using `subject.versions.first.end_with?('version1')` would be a little more readable.