Can't use `require` method for importing modules
This is likely an issue with my own babel setup, but when using an external npm module (in this case [xml2js](#)), I'm getting the following error:

```
<Error>: ReferenceError: Can't find variable: require.
	Plugin “[Plugin Name]”, line 47:
	» var Util = _interopDefault(require('util'));
2017-04-06 15:57:02.173 Sketch[15619:8424183] Exception: {
    column = 35;
    line = 47;
}
```

This same error happens any time I use a module that uses the `require` method of importing modules. Here's my `.babelrc`:

```
{
  presets: [
    ["es2015", {modules: false}],
    "react",
  ],
  plugins: [
    "external-helpers",
    "transform-react-jsx",
  ],
}
```

I imagine I'm missing something obvious here... the `es2015` preset should do the conversion, shouldn't it?

Thanks.