snapshot must be flagged current before update_versionless is called in Snapshot.set_current()

If any non-current snapshot is set as current using `tactic_client_lib.TacticServerStub.set_current_snapshot()`, the current versionless snapshot is not updated. This because none of the snapshots are flagged as `*is_current*` when `Snapshot.update_versionless` is called. 

This pull request suggests that `my` snapshot should be flagged as current before calling `update_versionless`.