title: Error in Laravel-mix
body: <p>I install Laravel-mix with npm and node on VPS </p>
  
  <p>OS is Ubuntu 16.4</p>
  
  <p>Laravel version is: 5.6</p>
  
  <p>npm version is : 3.5.2</p>
  
  <p>node version is: v4.2.6</p>
  
  <p>After run <code>sudo npm run production</code></p>
  
  <p>show this error:</p>
  
  <pre><code>production /var/www/html
  cross-env NODE_ENV=production node_modules/webpack/bin/webpack.js --
  no-progress --hide-modules --config=node_modules/laravel-
  mix/setup/webpack.config.js
  /var/www/html/node_modules/laravel-mix/setup/webpack.config.js:6
  let mix = require('../src/index');
  ^^^
  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:374:25)
  at Object.Module._extensions..js (module.js:417:10)
  at Module.load (module.js:344:32)
  at Function.Module._load (module.js:301:12)
  at Module.require (module.js:354:17)
  at require (internal/module.js:12:17)
  at requireConfig (/var/www/html/node_modules/webpack/bin/convert-argv.js:97:18)
  at /var/www/html/node_modules/webpack/bin/convert-argv.js:104:17
  at Array.forEach (native)
  
  npm ERR! Linux 4.4.0-87-generic
  npm ERR! argv "/usr/bin/nodejs" "/usr/bin/npm" "run" "production"
  npm ERR! node v4.2.6
  npm ERR! npm  v3.5.2
  npm ERR! code ELIFECYCLE
  npm ERR! @ production: `cross-env NODE_ENV=production 
  node_modules/webpack/bin/webpack.js --no-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 'cross-env 
  NODE_ENV=production node_modules/webpack/bin/webpack.js --no-progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js'.
  npm ERR! Make sure you have the latest version of node.js and npm 
  installed.
  npm ERR! If you do, 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!     cross-env NODE_ENV=production         node_modules/webpack/bin/webpack.js --no-progress --hide-modules --    config=node_modules/laravel-mix/setup/webpack.config.js
  npm ERR! You can get information on how to open an issue for this     project with:
  npm ERR!     npm bugs 
  npm ERR! Or if that isn't available, 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!     /var/www/html/npm-debug.log
  </code></pre>
  
  <p>How to issue this problem?</p>