Validate if github_token exists

Validate if the github_token session key exists before call it

PHPCI/Controller/SessionController.php

@@ -70,8 +70,8 @@ class SessionController extends b8\Controller
     */
     public function logout()
     {
-        unset($_SESSION['user_id']);
-        unset($_SESSION['github_token']);
+        $_SESSION = array();
+        session_destroy();
         header('Location: /');
         die;
     }