Fixed convertPlistToJson crashing when trying to parse data from stream

This is due to the fact that it was using `plist.parseFileSync` to parse the data incoming from the stream, but that data is actually just text. So this was crashing

```js
fs.createReadStream('test.plist', { encoding='utf-8' })
  .pipe(converter({
    from: 'plist',
    to: 'json'
  })
  .pipe(fs.createWriteStream('test.json', { encoding='utf-8' });
```

which is basically the example given in the README.

I changed it to use `plist.parseStringSync` instead, which works fine.

James