Here is the change to make in src/TwitterStatusUpdate.php within getRequestBody() function:
if ($this->inReplyToTweetId) {
$body['reply']['in_reply_to_tweet_id'] = (string) $this->inReplyToTweetId;
}
instead of
if ($this->inReplyToTweetId) {
$body['in_reply_to_tweet_id'] = $this->inReplyToTweetId;
}