Replace grunt-nsp-shrinkwrap with grunt-nsp

[pdehaan]

I think we can use something like this:

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

module.exports = function (grunt) {
  'use strict';

  grunt.config('nsp', {
    output: 'summary',
    shrinkwrap: grunt.file.readJSON('npm-shrinkwrap.json'),
  });
};