deps: update to webpack 4

package.json

     "karma-jasmine": "1.1.2",
     "karma-webpack": "3.0.0",
     "precommit-hook": "3.0.0",
-    "string-replace-loader": "1.3.0",
-    "uglifyjs-webpack-plugin": "1.2.2",
-    "webpack": "3.9.1"
+    "string-replace-loader": "2.1.1",
+    "webpack": "4.26.1",
+    "webpack-cli": "3.1.2"
   },
   "scripts": {
     "lint": "eslint . && flow",

webpack.config.js

 /* global __dirname */
 
 const process = require('process');
-const UglifyJsPlugin = require('uglifyjs-webpack-plugin');
-const webpack = require('webpack');
 
 const minimize
     = process.argv.indexOf('-p') !== -1
         || process.argv.indexOf('--optimize-minimize') !== -1;
-const plugins = [
-    new webpack.LoaderOptionsPlugin({
-        debug: !minimize,
-        minimize
-    })
-];
-
-if (minimize) {
-    plugins.push(new webpack.optimize.ModuleConcatenationPlugin());
-    plugins.push(new UglifyJsPlugin({
-        cache: true,
-        extractComments: true,
-        parallel: true,
-        sourceMap: true
-    }));
-}
 
 const config = {
     devtool: 'source-map',
+    mode: minimize ? 'development' : 'production',
     module: {
         rules: [ {
             // Version this build of the lib-jitsi-meet library.
         // value that is a mock (/index.js).
         __filename: true
     },
+    optimization: {
+        concatenateModules: minimize
+    },
     output: {
         filename: `[name]${minimize ? '.min' : ''}.js`,
+        path: process.cwd(),
         sourceMapFilename: `[name].${minimize ? 'min' : 'js'}.map`
-    },
-    plugins
+    }
 };
 
 module.exports = [