Webpack not updating on Heroku rebuild

I have a Node app which builds React with Webpack and is hosted on Heroku. Whenever I push a newer version to Heroku master, the React files do not update. I have now pushed several newer versions but the React files in webpack:// will not update and remain the originals from when I first deployed the app.

Here is my webpack.config.js file:

const webpack = require('webpack');
const path = require('path');

module.exports = {
 entry: {
  main: `${__dirname}/src/app.js`
 },
 output: {
  path: __dirname,
  filename: './public/bundle.js'
 },
 module: {
  loaders: [{
    loader: 'babel-loader',
    query: {
      presets: ['react', 'es2015', 'stage-2']
    },
    test: /\.jsx?$/,
    exclude: /(node_modules|bower_components)/
  }]
 },
 devtool: 'cheap-module-eval-source-map'
};

My package.json includes "heroku-postinstall": "webpack -p -w --config ./webpack.config.js --progress".

via Will