title: ExtractTextPlugin not working at webpack 2
body: <p>I am trying to migrate my webpack 1.x to 2.x but it is not working with ExtractTextPlugin.</p>
  
  <p>My webpack version is 2.1.0-beta.28 and "extract-text-webpack-plugin": "2.0.0-beta.4"</p>
  
  <p>Here is my webpack.config.js working without ExtractTextPlugin:</p>
  
  <pre><code>var path = require('path')
  var webpack = require('webpack')
  var HappyPack = require('happypack')
  var BundleTracker = require('webpack-bundle-tracker')
  var path = require('path')
  var ExtractTextPlugin = require("extract-text-webpack-plugin")
  
  function _path(p) {
    return path.join(__dirname, p);
  }
  
  module.exports = {
  
      context: __dirname,
      entry: {
          app: './assets/js/index',
          vendor: ['jquery', 'moment']
      }, 
  
      output: {
          path: path.resolve('./assets/bundles/'), 
          filename: '[name].js'
      },
  
      devtool: 'inline-eval-cheap-source-map',
  
      plugins: [
          new BundleTracker({filename: './webpack-stats.json'}), 
          new webpack.ProvidePlugin({ 
              $: 'jquery',
              jQuery: 'jquery',
              'window.jQuery': 'jquery' 
          }),
          new HappyPack({
              id: 'jsx',
              threads: 4,
              loaders: ["babel-loader"]
          }),
          // new ExtractTextPlugin({
          //     filename: "[name].css",
          //     disable: false,
          //     allChunks: true,
          // }),
          new webpack.optimize.CommonsChunkPlugin({name: "vendor", filename: "vendor.js"})
      ],
  
      module: {
          rules: [
  
               {
                  test: /\.css$/,
                  include: path.resolve(__dirname, './assets/css/'),
                  use: ["style-loader", "css-loader", "resolve-url-loader"]
                  // use: ExtractTextPlugin.extract({ fallbackLoader: "style-loader", loader: ["css-loader", "resolve-url-loader"]})
              },
  
              {
                  test: /\.scss$/,
                  include: path.resolve(__dirname, './assets/css/'),
                  use: ["style-loader", "css-loader", "resolve-url-loader", "sass-loader"]
                  // use: ExtractTextPlugin.extract({ fallbackLoader: "style-loader", loader: ["css-loader", "resolve-url-loader", "sass-loader"]})
              },
  
              {
                  test: /\.scss$/,
                  include: path.resolve(__dirname, './assets/vendors/'),
                  use: ["style-loader", "css-loader", "resolve-url-loader", "sass-loader"]
                  // use: ExtractTextPlugin.extract({ fallbackLoader: "style-loader", loader: ["css-loader", "resolve-url-loader", "sass-loader"]})
              },
  
              {
                  test: /\.jsx?$/, 
                  include: path.resolve(__dirname, './assets/js/'),
                  exclude: /node_modules/, 
                  use: ["happypack/loader?id=jsx"]
              },
              {
                  test: /\.png$/,
                  use: { 
                          loader: 'file-loader' ,
                          options: {
                              name: '/static/img/[name].[ext]'
                          }
                      }
  
              }
          ]
      },
  
      resolve: {
          extensions: ['.js', '.jsx'],
          modules: [ path.resolve(__dirname, 'node_modules')]
      }   
  }
  </code></pre>
  
  <p>So when I run webpack I got this error:
  Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
   - configuration.resolve has an unknown property 'modulesDirectories'. These properties are valid:
     object { alias?, aliasFields?, cachePredicate?, descriptionFiles?, enforceExtension?, enforceModuleExtension?, extensions?, fileSystem?, mainFields?, mainFiles?, moduleExtensions?, modules?, plugins?, resolver?, symlinks?, unsafeCache? }
   - configuration.resolve.extensions[0] should not be empty.</p>