Update grunt-contrib-jshint / Remove grunt.util._

* I updated [grunt-contrib-jshint](https://github.com/gruntjs/grunt-contrib-jshint) and fixed JSHint setting. Now there is no need to set the "ES5" option explicitly. (cf. http://www.jshint.com/blog/2013-05-07/2-0-0/).
* [`grunt.util._`](http://gruntjs.com/api/grunt.util#grunt.util._) is [deprecated](http://gruntjs.com/blog/2013-11-21-grunt-0.4.2-released), so I replaced it with [lodash](https://npmjs.org/package/lodash) module.