Laravel elixir problem

elixir.json

{
  "stylus": {
    "input": "./resources/area/frontend/stylus/app.styl",
    "output": "public/frontend/css/app.css"
  },
  "version": {
    "input": [
      "public/frontend/css/app.css"
    ],
    "output": "./public/frontend/build"
  },
  "browserSync": {
    "proxy": "vivid-blog.dev",
    "open": false
  }
}

gulpfile.js

var elixir = require('laravel-elixir'),
    config = require('./elixir.json');

require('laravel-elixir-stylus');

/*
 |--------------------------------------------------------------------------
 | Elixir Asset Management
 |--------------------------------------------------------------------------
 |
 | Elixir provides a clean, fluent API for defining some basic Gulp tasks
 | for your Laravel application. By default, we are compiling the Sass
 | file for our application, as well as publishing vendor resources.
 |
 */

elixir(function (mix) {
    mix.stylus(config.stylus.input, config.stylus.output)
        .version(config.version.input, config.version.output)
        .browserSync({
            proxy: config.browserSync.proxy,
            open: config.browserSync.open
        });
});