title: Tasks and sub-tasks not found
body: <p>I just started using Grunt and I already may have a problem. So, here is what my <code>Gruntfile.js</code> looks like :</p>
  
  <pre><code>module.exports = function (grunt) {
    grunt.initConfig({
  
      // Ability to access the package.json informations
      pkg: grunt.file.readJSON('package.json'),
  
      // SASS compilation
      sass: {
        dist: {
          options: {
            style: 'compressed'
          },
          expand: true,
          cwd: './sass/',
          src: 'main.scss',
          dest: './css/',
          ext: '.css'
        },
        dev: {
          options: {
            style: 'expanded',
            debugInfo: true,
            lineNumbers: true
          },
          expand: true,
          cwd: './sass/',
          src: 'main.scss',
          dest: './css/',
          ext: '.css'
        }
      }
  
    });
  };
  </code></pre>
  
  <p>And when I run <code>grunt sass:dev</code>, it always returns me <code>Warning: Task "sass:dev" not found. Use --force to continue.</code></p>
  
  <p>As I'm starting with it, I took a look at the doc and can't find where I may have gone wrong... And yes, dependencies are correctly installed.</p>