Allow numbers in project slugs so API driven builds work

Without this, a project whose slug is e.g. `api-v3` will kaboom at build time (at the Celery layer) because URL resolution finds the PK version of the URL (intended for `sync_versions`) and eventually tries querying the database for kwargs like `{'pk': 'api-v3'}`. This of course is invalid and causes a lookup error (which in turn is masking the core `ValueError` that happens when the ORM tries to `int('api-v3')`.)

I'm not 100% sure why this API endpoint is hit via Celery builds but not, apparently, user/WWW-driven builds. Either way it seems like a clear cut bug.