Made the Http Kernel class abstract

Added Abstract to the Http default Kernel.php, because I was getting this error and adding abstract to it seemed to get the app working again:

Error Output: PHP Fatal error:  Class Illuminate\Foundation\Http\Kernel contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Illuminate\Contracts\Http\Kernel::terminate) in /var/www/laravel/storage/framework/compiled.php on line 1236