Remove grunt.util._ / Update devDependencies / Use single quotes

* Replaced deprecated [`grunt.util._`](http://gruntjs.com/api/grunt.util#grunt.util._) with [lodash](https://www.npmjs.org/package/lodash) module
* Updated devDependencies 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/)
* Replaced all the double quotes with single quotes because [Grunt contribution guide](http://gruntjs.com/contributing#syntax) recommends using single quotes.