Issue #411: Getting child comment causes fatal error

`get_comment()` with the post ID doesn't work, changed `get_comment()` call in the `WP_JSON_Posts` `prepare_comment()` function to use the `$comment->comment_parent` field. 

See [lib/class-wp-json-posts.php#L1583](https://github.com/WP-API/WP-API/blob/master/lib/class-wp-json-posts.php#L1583)
See Issue #411