Doesn't work with untagged blueprints

https://github.com/ember-engines/engine-blueprint doesn't use any versioning, so it fails with ember-cli-update. We could possibly track commits instead of versions. That would mean we would have to check commit history to see if an update is needed.

Start by reproducing the failure in ember-cli-update with the untagged ember-engines/engine-blueprint repository. Inspect how the updater determines blueprint versions and what it checks to decide whether an update is needed. Done should include a defined way to detect updates for unversioned blueprints, with behavior verified against that repository.

Written by the indexing model from the issue text.