Fix broken SnapshotMetaSpec
Change Date arbitrary so it doesn't skip
SnapshotMeta.latest should take latest based of date/store pair, not just date
Take the greatest snapshot id when there are duplicate snapshots

- Added more test cases
- Added OldIdentifier arbitrary
Merge pull request #140 from ambiata/topic/fix-broken-smapshot-meta-test

Fix broken SnapshotMetaSpec