CI: added Symfony 4.0.0 environment and symfony-demo app included (#4681)

* CI: added Symfony 4.0.0 environment and symfony-demo app included

* fixed running different commands for different Symfony versions

* fixed travis tests

* fixed running

* Exlicitly setting suite for Symfony 4 tests

* dont instal symfony/symfony for demo app in symfony 4

* fixed version comparison

* Update .travis.yml

* Update .travis.yml

* fixed Symfony to load kernel from a different location

* removed --no-dev for symfony-demo install

* Update .travis.yml

* Update .travis.yml

Author: DavertMik

src/Codeception/Module/Symfony.php

@@ -256,8 +256,8 @@ public function _getContainer()
      */
     protected function getKernelClass()
     {
-        $path = \Codeception\Configuration::projectDir() . $this->config['app_path'];
-        if (!file_exists(\Codeception\Configuration::projectDir() . $this->config['app_path'])) {
+        $path = codecept_root_dir() . $this->config['app_path'];
+        if (!file_exists(codecept_root_dir() . $this->config['app_path'])) {
             throw new ModuleRequireException(
                 __CLASS__,
                 "Can't load Kernel from $path.\n"
@@ -277,6 +277,11 @@ protected function getKernelClass()
         }
         $file = current($results);
 
+        if (file_exists(codecept_root_dir() . 'vendor' . DIRECTORY_SEPARATOR . 'autoload.php')) {
+            // ensure autoloader from this dir is loaded
+            require_once codecept_root_dir() . 'vendor' . DIRECTORY_SEPARATOR . 'autoload.php';
+        }
+
         require_once $file;
 
         $possibleKernelClasses = [