title: new to grunt - warning: task "concat, uglify" not found
body: <p>As the title says I'm new to Grunt. I am following a tutorial located at: <a href="http://24ways.org/2013/grunt-is-not-weird-and-hard/" rel="nofollow">http://24ways.org/2013/grunt-is-not-weird-and-hard/</a>. It is an older tutorial but most seems to work the same. I have installed "grunt-contrib-concat" and "grunt-contrib-uglify" and can run both individually. But when I run <code>grunt</code>, I get the following error: <code>Warning: Task "concat, uglify" not found. Use --force to continue. Aborted due to errors.</code> I've been looking around and can't seem to figure it out. My files are as follows:</p>
  
  <p>Gruntfile.js:</p>
  
  <pre><code>module.exports = function(grunt) {
  
              // 1. All configuration goes here 
              grunt.initConfig({
                  pkg: grunt.file.readJSON('package.json'),
  
                  concat: {
  
                      dist: {
                          src: [
                              'js/libs/*.js', // All JS in the libs folder
                              'js/controls.js', // This specific file
                          ],
                          dest: 'dist/built.js',
                      }
                  },
  
                  uglify: {
                      build: {
                          src: 'js/build/production.js',
                          dest: 'js/build/production.min.js',
                      }
                  },
  
              });
  
              // 3. Where we tell Grunt we plan to use this plug-in.
              grunt.loadNpmTasks('grunt-contrib-concat');
              grunt.loadNpmTasks('grunt-contrib-uglify');
  
              // 4. Where we tell Grunt what to do when we type 'grunt' into the terminal.
              grunt.registerTask('default', ['concat, uglify']);
  
          };
  </code></pre>
  
  <p>package.json:</p>
  
  <pre><code>{
    "name": "grunt_libsass_example-project",
    "version": "0.1.0",
    "devDependencies": {
      "grunt": "~0.4.1",
      "grunt-contrib-concat": "^0.5.1",
      "grunt-contrib-uglify": "^0.9.1"
    }
  }
  </code></pre>