Removed retval prefix for cleaner outputs

Author: Andrés Correa Casablanca

src/Handlers/ShellMessagesHandler.php

@@ -16,10 +16,10 @@
 use Litipk\JupyterPHP\Actions\KernelInfoAction;
 use Litipk\JupyterPHP\Actions\ShutdownAction;
 use Litipk\JupyterPHP\JupyterBroker;
+use Litipk\JupyterPHP\Shell;
 
 use Litipk\JupyterPHP\KernelOutput;
 use Monolog\Logger;
-use Psy\Shell;
 use React\ZMQ\SocketWrapper;
 
 final class ShellMessagesHandler

src/KernelCore.php

@@ -11,7 +11,6 @@
 
 namespace Litipk\JupyterPHP;
 
-
 use Litipk\JupyterPHP\Handlers\HbErrorHandler;
 use Litipk\JupyterPHP\Handlers\HbMessagesHandler;
 use Litipk\JupyterPHP\Handlers\IOPubMessagesHandler;
@@ -23,7 +22,6 @@
 use React\ZMQ\Context as ReactZmqContext;
 use React\ZMQ\SocketWrapper;
 
-
 /**
  * Class KernelCore (no pun intended)
  */

src/Shell.php

@@ -0,0 +1,19 @@
+<?php
+
+/*
+ * This file is part of Jupyter-PHP.
+ *
+ * (c) 2015-2017 Litipk
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Litipk\JupyterPHP;
+
+use Psy\Shell as PsyShell;
+
+class Shell extends PsyShell
+{
+    const RETVAL = '';
+}