title: have isuue with using Pusher with laravel 5.6
body: <p>I'm trying to use pusher with laravel 5.6 and vue.js but i'm always getting error message</p>
  
  <blockquote>
    <p>uncaught exception: You must pass your app key when you instantiate Pusher.</p>
  </blockquote>
  
  <p>i already created free pusher account and i posted my appkey at .env file</p>
  
  <pre><code>PUSHER_APP_ID=&lt;REDACTED&gt; PUSHER_APP_KEY=&lt;REDACTED&gt;
  PUSHER_APP_SECRET=&lt;REDACTED&gt; PUSHER_APP_CLUSTER=eu
  
  MIX_PUSHER_APP_KEY="${PUSHER_APP_KEY}" MIX_PUSHER_APP_CLUSTER="${PUSHER_APP_CLUSTER}"
  </code></pre>
  
  <p>and at bootstrab.js i already created new echo instance and posted my pusher settings like this</p>
  
  <pre><code>import Echo from 'laravel-echo' window.Pusher = require('pusher-js');
  
  window.Echo = new Echo({ broadcaster: 'pusher', key:process.env.MIX_PUSHER_APP_KEY, cluster:process.env.MIX_PUSHER_APP_CLUSTER, encrypted: true });
  
  pusher.log=function(message) { window.console.log(message) }
  </code></pre>
  
  <p>can some body help to solve this issue please...</p>