Expose version retrieval timestamp on HTTP API

[gasinvein]

The "Retrieved on (UTC)" field could be useful for HTTP API users, who may want to know when the version was released without further crawling the upstream project, if Anytia provides this information.

[Zlopez]

It should be easy to add this field to API.

[gasinvein]

versions and stable_version are plain string lists - I guess timestamps can't be added there, so we'll need a new field to contain a list of (version, retrieved_on) pairs?

[Zlopez]

You are right, this would be probably a backwards incompatible change, because you need to change the type to object from list.