deprecation issue in data event example in readme

[Frijol]

running the still image on 'data' event from the readme:

'use strict';
const fs = require('fs');
const path = require('path');

const av = require('tessel-av');
const camera = new av.Camera();
const capture = camera.capture();

capture.on('data', function(data) {
  fs.writeFile(path.join(__dirname, 'captures/captured-via-data-event.jpg'), data);
});

creates this error

(node:1897) [DEP0013] DeprecationWarning: Calling an asynchronous function without callback is deprecated.
fs.js:113
      throw err;  // Forgot a callback but don't know where? Use NODE_DEBUG=fs
      ^