Fix bot.replyPrivateDelayed. howdyai#444

I discovered this when trying to ephemerally respond to an interactive button click. We need to set `replace_original` to `false` in order for our reply to be ephemeral. Otherwise, it will reply publicly.

lib/Slackbot_worker.js

@@ -494,6 +494,7 @@ module.exports = function(botkit, config) {
             msg.channel = src.channel;
 
             msg.response_type = 'ephemeral';
+            msg.replace_original = false;
 
             var requestOptions = {
                 uri: src.response_url,