Update jsoncpp to fix broken reference commit

The reference commit open-source-parsers/jsoncpp@e900f5b91249fdce24493fe731c2f6d15dbed4b5 is not on `master` and causes a problem on checkout:

```
$ git submodule update --init
fatal: reference is not a tree: e900f5b91249fdce24493fe731c2f6d15dbed4b5
Unable to checkout 'e900f5b91249fdce24493fe731c2f6d15dbed4b5' in submodule path 'thirdparty/jsoncpp'
```

This pull request updates it to the `master` `HEAD`.