Fix missing sliced_for_model migration (Issue #211)
The sliced_for_model column was added to the PrintArchive model for
model-based queue assignment but the database migration was missing.
This caused upgrades from 0.1.6b11 to 0.1.6 to fail with:
"no such column: print_archives.sliced_for_model"
Users' data was not deleted - just inaccessible due to query failures.