Don't error when the file size is too large, just skip

This is to fix issue #6. If the error code response from pngquant is for the file being too large, ignore it and carry on execution.
Just kidding, use this instead:

```js
if (err) {
    if (err.code === 98) {
        cb(null, file);
        return
    }

    cb(err)
    return;
}
```