When trying to initialize Amplitude Classic or Madkudu in a website using SystemJS, the initialization fails with the following message: ``` analytics.min.js:1 Madkudu TypeError: window.require is not a function ``` This is due to the fact that the condition on [this line](https://github.com/segmentio/analytics.js-integrations/blob/548c10c110f9514b3d8a84ef1f1db8a5ff81e488/integrations/madkudu/lib/index.js#L13) is true for SystemJS, but SystemJS does not define `window.require` (see [here](https://github.com/systemjs/systemjs/issues/1990#issuecomment-523992485) and [here](https://github.com/systemjs/systemjs/issues/2296#issuecomment-778423843)). Maybe to [the condition](https://github.com/segmentio/analytics.js-integrations/blob/548c10c110f9514b3d8a84ef1f1db8a5ff81e488/integrations/madkudu/lib/index.js#L13) on line 13 a check should be added for `window.require` to be defined.