Update example to use ES6 syntax

[mattbaileyuk]

A small update so that the example nodeJS application uses ES6 syntax (now jsonata is node v6+).

[andrew-coleman]

@mattbaileyuk fyi, there will be an imminent PR containing the new docs. It will become part of the jsonata repo and deployed by travis for every merge into master. Will need to apply this patch to:
https://github.com/andrew-coleman/jsonata/blob/master/docs/using-nodejs.md