title: Webpack config object validation error: "loader" must be a string
body: <p>I'm using Webpack Validator to validate my Webpack Config object and it's giving me the following error:</p>
  <pre><code>[1] &quot;loader&quot; must be a string
  </code></pre>
  <p>Here is my Webpack config object:</p>
  <pre class="lang-js prettyprint-override"><code>const config = webpackValidator({
    context: resolve('client'),
    entry: {
      app: './app.js',
      vendor: ['./app.css'],
    },
    output: {
      filename: ifProd('bundle.[name].[chunkhash].js', 'bundle.[name].js'),
      path: resolve('dist'),
      pathinfo: ifNotProd(),
    },
    devtool: ifProd('source-map', 'eval'),
    module: {
      loaders: [
        {test: /\.js$/, loaders: ['babel-loader'], exclude: /node_modules/},
        {test: /\.jsx$/, loaders: ['babel-loader'], exclude: /node_modules/},
        {
          test: /\.css$/, loader: ExtractTextPlugin.extract({
            fallbackLoader: 'style-loader',
            loader: 'css-loader'
          })
        },
      ],
    },
    plugins: removeEmpty([
      new ProgressBarPlugin(),
      new ExtractTextPlugin(ifProd('styles.[name].[chunkhash.css]', 'styles.[name].css')),
      ifProd(new InlineManifestWebpackPlugin()),
      ifProd(new webpack.optimize.CommonsChunkPlugin({
        name: ['vendor', 'manifest'],
      })),
      new HtmlWebpackPlugin({
        template: './index.html',
        inject: 'head',
      }),
    ]),
  })
  </code></pre>
  <p>Any insight would be greatly appreciated!</p>