Error Deploying React app on heroku

Im trying to deploy a React app on heeroku and in browser, I keep getting this message

```
Failed to compile.

Error in multi main
Module not found: 'eslint' in /app

 @ multi main
```

In heroku logs it says

2017-04-15T05:02:24.851322+00:00 app[web.1]: Failed to compile.
2017-04-15T05:02:24.851371+00:00 app[web.1]:
2017-04-15T05:02:24.851475+00:00 app[web.1]: Error in multi main
2017-04-15T05:02:24.851477+00:00 app[web.1]:
2017-04-15T05:02:24.851476+00:00 app[web.1]: Module not found: 'eslint' in /app
2017-04-15T05:02:24.851478+00:00 app[web.1]:  @ multi main
2017-04-15T05:02:24.851525+00:00 app[web.1]:

I am not sure what is going, I did npm install eslint --save-dev

and my webpack preloader references eslint but it does not work

//webpack.config.dev.js file

var autoprefixer = require('autoprefixer');
var webpack = require('webpack');
var HtmlWebpackPlugin = require('html-webpack-plugin');
var CaseSensitivePathsPlugin = require('case-sensitive-paths-webpack-plugin');
var InterpolateHtmlPlugin = require('react-dev-utils/InterpolateHtmlPlugin');
var WatchMissingNodeModulesPlugin = require('react-dev-utils/WatchMissingNodeModulesPlugin');
var getClientEnvironment = require('./env');
var paths = require('./paths');


var publicPath = '/';

var publicUrl = '';
var env = getClientEnvironment(publicUrl);

module.exports = {
  devtool: 'cheap-module-source-map',
  entry: [
    require.resolve('react-dev-utils/webpackHotDevClient'),
    require.resolve('./polyfills'),
    paths.appIndexJs
  ],
  output: {
    // Next line is not used in dev but WebpackDevServer crashes without it:
    path: paths.appBuild,
    // Add /* filename */ comments to generated require()s in the output.
    pathinfo: true,
    // This does not produce a real file. It's just the virtual path that is
    // served by WebpackDevServer in development. This is the JS bundle
    // containing code from all our entry points, and the Webpack runtime.
    filename: 'static/js/bundle.js',
    // This is the URL that app is served from. We use "/" in development.
    publicPath: publicPath
  },
  resolve: {
    // This allows you to set a fallback for where Webpack should look for modules.
    // We read `NODE_PATH` environment variable in `paths.js` and pass paths here.
    // We use `fallback` instead of `root` because we want `node_modules` to "win"
    // if there any conflicts. This matches Node resolution mechanism.
    // https://github.com/facebookincubator/create-react-app/issues/253
    fallback: paths.nodePaths,
    // These are the reasonable defaults supported by the Node ecosystem.
    // We also include JSX as a common component filename extension to support
    // some tools, although we do not recommend using it, see:
    // https://github.com/facebookincubator/create-react-app/issues/290
    extensions: ['.js', '.json', '.jsx', ''],
    alias: {
      // Support React Native Web
      // https://www.smashingmagazine.com/2016/08/a-glimpse-into-the-future-with-react-native-for-web/
      'react-native': 'react-native-web'
    }
  },

  module: {
    // First, run the linter.
    // It's important to do this before Babel processes the JS.
    preLoaders: [
      {
        test: /\.(js|jsx)$/,
        loader: 'eslint',
        include: paths.appSrc,
      }
    ],
    loaders: [
      // ** ADDING/UPDATING LOADERS **
      // The "url" loader handles all assets unless explicitly excluded.
      // The `exclude` list *must* be updated with every change to loader extensions.
      // When adding a new loader, you must add its `test`
      // as a new entry in the `exclude` list for "url" loader.

      // "url" loader embeds assets smaller than specified size as data URLs to avoid requests.
      // Otherwise, it acts like the "file" loader.
      {
        exclude: [
          /\.html$/,
          /\.(js|jsx)$/,
          /\.css$/,
          /\.json$/,
          /\.svg$/
        ],
        loader: 'url',
        query: {
          limit: 10000,
          name: 'static/media/[name].[hash:8].[ext]'
        }
      },
      // Process JS with Babel.
      {
        test: /\.(js|jsx)$/,
        include: paths.appSrc,
        loader: 'babel',
        query: {

          // This is a feature of `babel-loader` for webpack (not Babel itself).
          // It enables caching results in ./node_modules/.cache/babel-loader/
          // directory for faster rebuilds.
          cacheDirectory: true
        }
      },
      // "postcss" loader applies autoprefixer to our CSS.
      // "css" loader resolves paths in CSS and adds assets as dependencies.
      // "style" loader turns CSS into JS modules that inject <style> tags.
      // In production, we use a plugin to extract that CSS to a file, but
      // in development "style" loader enables hot editing of CSS.
      {
        test: /\.css$/,
        loader: 'style!css?importLoaders=1!postcss'
      },
      // JSON is not enabled by default in Webpack but both Node and Browserify
      // allow it implicitly so we also enable it.
      {
        test: /\.json$/,
        loader: 'json'
      },
      // "file" loader for svg
      {
        test: /\.svg$/,
        loader: 'file',
        query: {
          name: 'static/media/[name].[hash:8].[ext]'
        }
      }
      // ** STOP ** Are you adding a new loader?
      // Remember to add the new extension(s) to the "url" loader exclusion list.
    ]
  },

  // We use PostCSS for autoprefixing only.
  postcss: function() {
    return [
      autoprefixer({
        browsers: [
          '>1%',
          'last 4 versions',
          'Firefox ESR',
          'not ie < 9', // React doesn't support IE8 anyway
        ]
      }),
    ];
  },
  plugins: [
    new InterpolateHtmlPlugin(env.raw),
    new HtmlWebpackPlugin({
      inject: true,
      template: paths.appHtml,
    }),
    new webpack.DefinePlugin(env.stringified),
    new webpack.HotModuleReplacementPlugin(),
    new CaseSensitivePathsPlugin(),
    new WatchMissingNodeModulesPlugin(paths.appNodeModules)
  ],
  node: {
    fs: 'empty',
    net: 'empty',
    tls: 'empty'
  }
};

via henhen