ظهور خطأ عند تنفيذ npm run production في لارافل

عندما أقوم بتنفيذ الأمر: npm run production في مشروع لارافل يظهر لدي الخطأ التالي:

Block-scoped declarations not yet supported

وهذا هو النص الكامل للخطأ:

> @ production /home/vagrant/myproject/pr1
> node node_modules/cross-env/bin/cross-env.js NODE_ENV=production node_modules/webpack/bin/webpack.js --progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js

/home/vagrant/myproject/pr1/node_modules/laravel-mix/setup/webpack.config.js:120
        let extractPlugin = new plugins.ExtractTextPlugin(
        ^^^

SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:404:25)
    at Object.Module._extensions..js (module.js:432:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:311:12)
    at Module.require (module.js:366:17)
    at require (module.js:385:17)
    at requireConfig (/home/vagrant/projects/nielsvroman/node_modules/webpack/bin/convert-argv.js:96:18)
    at /home/vagrant/projects/nielsvroman/node_modules/webpack/bin/convert-argv.js:109:17
    at Array.forEach (native)

npm ERR! Linux 3.19.0-25-generic
npm ERR! argv "/usr/bin/nodejs" "/usr/bin/npm" "run" "production"
npm ERR! node v5.0.0
npm ERR! npm  v3.3.6
npm ERR! code ELIFECYCLE
npm ERR! @ production: `node node_modules/cross-env/bin/cross-env.js NODE_ENV=production node_modules/webpack/bin/webpack.js --progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the @ production script 'node node_modules/cross-env/bin/cross-env.js NODE_ENV=production node_modules/webpack/bin/webpack.js --progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js'.
npm ERR! This is most likely a problem with the  package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     node node_modules/cross-env/bin/cross-env.js NODE_ENV=production node_modules/webpack/bin/webpack.js --progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js
npm ERR! You can get their info via:
npm ERR!     npm owner ls
npm ERR! There is likely additional logging output above.

npm ERR! Please include the following file with any support request:
npm ERR!     /home/vagrant/myproject/pr1/npm-debug.log

والكود الموجود حالياً في ملف webpack.min.js:

const { mix } = require('laravel-mix');

/*
 |--------------------------------------------------------------------------
 | Mix Asset Management
 |--------------------------------------------------------------------------
 |
 | Mix provides a clean, fluent API for defining some Webpack build steps
 | for your Laravel application. By default, we are compiling the Sass
 | file for the application as well as bundling up all the JS files.
 |
 */

mix.js([
        'resources/assets/plugins/jquery-1.11.3.min.js',
        'resources/assets/plugins/bootstrap/js/bootstrap.min.js',
        'resources/assets/js/main.js'
    ], 'public/js'
);

mix.combine([
       'resources/assets/plugins/bootstrap/css/bootstrap.min.css',
       'resources/assets/plugins/font-awesome/css/font-awesome.css',
       'resources/assets/css/styles.css'
   ], 'public/css/all.css');

كيف يمكنني حل المشكلة؟