Use Object.create instead of grunt.util._.clone

Because [grunt.util._ is deprecated](http://gruntjs.com/api/grunt.util#grunt.util._).

Basically, [Object.create](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Object/create) and [_.clone](http://lodash.com/docs#clone) work differently and cannot be replaced with each other. However, they will do almost the same work in this case.
Besides, [Object.create is faster than _.clone](http://jsperf.com/object-create-vs-lodash-clone).