Parent of dependency should be empire (not country)

[nvkelso]

In updating whosonfirst-data/whosonfirst-data#763 we've noted that dependencies should be equivalent to country in that they are both parented by empires. But we need to update the JSON and docs to that effect. (Pelias already treats them as equivalent.)

• JSON: https://github.com/whosonfirst/whosonfirst-placetypes/blob/master/placetypes/dependency.json#L5
• dependency definition on https://github.com/whosonfirst/whosonfirst-placetypes/blob/master/README.md
• image: https://github.com/whosonfirst/whosonfirst-placetypes/blob/master/images/placetypes-latest.png