code: function (id, version, done, config) {
          if (!_.isNumber(parseInt(id, 10))) {
            return done(new Error('id must be specified.'));
          }
          if (!_.isEmpty(version) && !_.isNumber(parseInt(version, 10))) {
            return done(new Error('version must be a number.'));
          }
  
          var payload = {
            type: 'file_version',
            id: version
          };
          this._request(['files', id, 'versions', 'current'], 'POST', done, null, payload, null, null, null, config);
        }
docs: If there are previous versions of this file, this method can be used to promote one of the
  older versions to the top of the stack. This actually mints a copy of the old version and
  puts it on the top of the versions stack. The file will have the exact same contents, the
  same SHA1/etag, and the same name as the original. Other properties such as comments do not
  get updated to their former values.
  @summary Promote an Old Version of a File.
  @see {@link https://developers.box.com/docs/#files-promote-old-version}
  @param {number} id - The file's ID.
  @param {number} version - File version to promote.
  @param {requestCallback} done - The callback to invoke (with possible errors) when the request returns.
  @param {?RequestConfig} [config] - Configure the request behaviour.