Ensure that rotation cache has proper indices

As part of UW-Macrostrat/macrostrat#55, we had to ensure that the `corelle.rotation_cache` index was unique – it appears to have been generated against plate polygons instead of the plates themselves.

```
ALTER TABLE corelle.rotation_cache ADD CONSTRAINT rotation_cache_unique UNIQUE (model_id, plate_id, t_step);
```

We should ensure that this index gets added on table creation.