cd /var/www
mkdir slackgun
cd slackgun
npm init
..
npm install express --save
npm install node-slack-mailgun --save

// app.js
var SlackGun = require('node-slack-mailgun');
var slack_hook_url = 'foo';
var mailgun_apikey = 'bar';
var express = require('express');

var app = express();

app.use('/api/mailgun', SlackGun({
    slack: {
        hook: slack_hook_url,
        channel: 'dev_bad_notifications'
    },
    mailgun: { apikey: mailgun_apikey }
}));

$ node app.js

/var/www/slackgun/node_modules/node-slack-mailgun/index.js:31
        that.options = getOptions(options);
                     ^
TypeError: Cannot set property 'options' of undefined
    at SlackGun (/var/www/slackgun/node_modules/node-slack-mailgun/index.js:31:15)
    at Object.<anonymous> (/var/www/slackgun/app.js:8:25)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:929:3

  var json = msg.payload.reply;
  if (json.attachment) {
      var attachment_array = JSON.stringify(json.attachment)
      var slack = global.get("slack");
      msg.payload = slack.api.test(JSON.stringify(msg.payload.reply), console.log)
  }