Error trying to send Email using Mailgun in Laravel 5.1

I have a problem when I try to send emails with Mailgun, I already configured my project according the documentation in the page on Mailgun and some tutorials, but the error still there, I hope someone help's me!!

Here is the error:

Mailgun on Laravel 5.1 error

Here is the code:

Env configuration

MAIL_DRIVER= mailgun
MAILGUN_DOMAIN= sandbox***************.mailgun.org
MAILGUN_PASS= key-********************************

Services configuration

'mailgun' => [
    'domain' => env('MAILGUN_DOMAIN'),
    'secret' => env('MAILGUN_PASS'),
]

Controller

$data = [];
    \Mail::send('emails.plantilla', $data, function($message){
        $message->from('some@gmail.com')
                ->to('aguilas@hotmail.com')
                ->subject('Probando mailgun en laravel 5.1');
    });

via Chebli Mohamed