Do not call meta.SetStored().
Fix the signature of SetStored.
Add a doc comment.
Adjust to further changes in {utils}/filestorage.
Move interface checks into a test file.
Adjust a doc comment.
Dereference values consistently.
Clarify an error message.
Test against a fake backups storage.
Adjust to metadata changes.
Set the resulting metadata ID after creating a backup.
Update utils repo dependency.
Merge pull request #690 from ericsnowcurrently/backups-fix-SetStored

Adjust to an API change over in utils/filestorage.

Once https://github.com/juju/utils/pull/33 lands, this patch will include updating dependencies.tsv as well.