Logging: use %s to format the variable
Using %d was causing an issue when trying to format `None`:
```
celery-web celery.redirected:227[19247]: WARNING Message: '(Sync Versions) Deleted Versions: project=%s versions_count=[%d]'
celery-web Arguments: ('***************', None)
```
This commit uses %s and defaults to 0 when no versions were deleted.
