Appcues fix/appcues

@calvinfo seems good to me https://github.com/segmentio/analytics.js-integrations/pull/193
I feel like this might be a bit nicer if we init window.Appcues directly after load.
```js
var script = load(url, function(){
  window.Appcues.init();
  callback();
});
```

then we wouldn't have to call init elsewhere.