Return array of version uris

[awead]

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

[awead]

Don't merge this yet. I need to add a test case for fixity checking of each version.

[jcoyne]

how about version_uri because we are returning the URI, not the version metadata or content.

[coveralls]

Coverage remained the same when pulling ed4547f on versions into 06d405b on fedora-4.

[coveralls]

Coverage increased (+0.01%) when pulling 39e55de on versions into 06d405b on fedora-4.

[awead]

@jcoyne Travis is passing now and I've updated this PR per your comments.