message: fix error when message.delivery.mids is undefined
diff: @@ -15,9 +15,11 @@ function Facebookbot(configuration) {
  facebook_botkit.on('message_delivered', function(bot, message) {
  + var deliveryMids = message.delivery && message.delivery.mids || [];
  +
  // get list of mids in this message
  - for (var m = 0; m < message.delivery.mids.length; m++) {
  - var mid = message.delivery.mids[m];
  + for (var m = 0; m < deliveryMids.length; m++) {
  + var mid = deliveryMids[m];
  // loop through all active conversations this bot is having
  // and mark messages in conversations as delivered = true
  