Update dependencies and devDependencies

* `grunt.util.async` is [deprecated](http://gruntjs.com/api/grunt.util#grunt.util.async), so I replaced it with [async](https://github.com/caolan/async) module.
* I 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/).
* I moved Blanket configuration in `package.json` from the `scripts` field to the `config` field. (cf. alex-seville/blanket#349)