Run code sniffer also in plugin root directory

[MAILPOET-2429]

RoboFile.php

@@ -1,5 +1,6 @@
 <?php
 
+// phpcs:ignore PSR1.Classes.ClassDeclaration
 class RoboFile extends \Robo\Tasks {
   const ZIP_BUILD_PATH = __DIR__ . '/mailpoet.zip';
 
@@ -377,6 +378,11 @@ class RoboFile extends \Robo\Tasks {
         'lib',
         'tests',
       ])
+
+      // PHP >= 5.6 in plugin root directory
+      ->taskExec($task)
+      ->rawArg('--runtime-set testVersion 5.6-7.3')
+      ->rawArg('-l .')
       )
       ->run();
   }

mailpoet.php

@@ -15,13 +15,13 @@
  * @since 3.0.0-beta.1
  */
 
-$mailpoet_plugin = array(
+$mailpoet_plugin = [
   'version' => '3.38.0',
   'filename' => __FILE__,
   'path' => dirname(__FILE__),
   'autoloader' => dirname(__FILE__) . '/vendor/autoload.php',
-  'initializer' => dirname(__FILE__) . '/mailpoet_initializer.php'
-);
+  'initializer' => dirname(__FILE__) . '/mailpoet_initializer.php',
+];
 
 function mailpoet_deactivate_plugin() {
   deactivate_plugins(plugin_basename(__FILE__));