REGRESSION: clamp NE roads to min_zoom 5

Following up from https://github.com/tilezen/vector-datasource/pull/1352#discussion_r130407407...

Looks like we source the Natural Earth scale_rank for min_zoom directly, and that starts at 3:

```
    min_zoom: { col: scalerank }
```

So the changes in #1352 would introduce roads at zoom 3. We need to clamp the Natural Earth roads to 5.