adding `readyOnLoad` fix to still call `load` callback

I had the problem where one of my integrations wasn't marked `readyOnLoad()`
and instead did some setup after the load callback before emitting `ready`.

```js
Integration.prototype.initialize = function () {
  var self = this;
  this.load(function () {
    // ...
    self.emit('ready');
  });
};
```

This fixes the load callback so that it will actually get called.