Only index latest charm version.

Use elasticsearch versioning to make sure that only
the latest version of the charm or bundle is indexed.
Therefore only returning the latest version in search
results.
That's a reasonable question.
I'd be tempted to make this just return error, and define an error value, say:

   var ErrConflict = errgo.New("conflict inserting document")

and document that PutDocumentVersion returns this error on conflict.