title: Grunt usemin issue: unable to process the task.required config property concat.generated missing
body: <p>I have a folder structure like this</p>
  
  <pre><code>app
  |
  +- html
  |   +- index.html
  +- assets
  |   +- js
  |      +- foo.js
  |      +- bar.js
  +- dist
  </code></pre>
  
  <p>My Gruntfile.js is</p>
  
  <pre><code>module.exports = function (grunt) {
    grunt.initConfig({
      config: {
        app: 'app',
        dist: 'dist'
      },
      useminPrepare: {
        html: 'html/index.html',
        options: {
          root: '&lt;%= config.app %&gt;',
          dest: '&lt;%= config.dist %&gt;'
        }
      },
      usemin: {
        html: ['&lt;%= config.dist %&gt;/html/index.html'],
        js: '&lt;%= config.dist %&gt;/assets/js/*.js'
      }
    });
  
    grunt.loadNpmTasks('grunt-contrib-clean');
    grunt.loadNpmTasks('grunt-rev');
    grunt.loadNpmTasks('grunt-usemin');
    grunt.loadNpmTasks('grunt-contrib-copy');
    grunt.loadNpmTasks('grunt-contrib-concat');
    grunt.loadNpmTasks('grunt-contrib-cssmin');
    grunt.loadNpmTasks('grunt-contrib-uglify'); 
  
    grunt.registerTask('build', [
       'useminPrepare',
       'concat:generated',
       'cssmin:generated',
       'uglify:generated',
       'usemin'
    ]);
  };
  </code></pre>
  
  <p>in html/index.html</p>
  
  <pre><code>&lt;!-- build:js /assets/js/optimized.js --&gt;
  &lt;script src="/assets/js/foo.js"&gt;&lt;/script&gt;
  &lt;script src="/assets/js/bar.js"&gt;&lt;/script&gt;
  &lt;!-- endbuild --&gt;
  </code></pre>
  
  <p>When I execute the 'grunt build' command, it shows "unable to process the task.required config property concat.generated missing" error.
  What could be the reason?</p>