cleanup() breaks.

I was enjoying `jsonp()` when i noticed `cleanup()` breaks because it attempts to get a `.parentNode` of a script that doesn't exist.

You can reproduce this with:

```js

var jsonp = require('jsonp');

jsonp('https://www.googleapis.com/freebase/v1/text/en/bob_dylan', function(){
  console.log(arguments);
});

jsonp('https://www.googleapis.com/freebase/v1/text/en/bob_dylan', function(){
  console.log(arguments);
});

```

One of those will fail to execute with the error `Uncaught TypeError: Cannot call method 'removeChild' of null `