Module not compatible with 8.6.x

Drupal core 8.6.x moves hierarchy from {taxonomy_term_hierarchy} to {taxonomy_term__parent} in taxonomy_update_8502. At the same time, we have some direct db queries in taxonomy_term_depth_get_parent() and taxonomy_term_depth_get_child() querying {taxonomy_term_hierarchy}.

Proposed resolution:
Use some core API instead of querying the db.