Laravel Collective does not work

I am trying to learn Laravel using the Laracasts and it's very interesting. In the step of creating forms, I would like to use Laravel Collective. So I went to the website http://ift.tt/1NsyIda and tried what's written, but I get the following error:

FatalErrorException in ProviderRepository.php line 146:
Class 'Collective\Html\HtmlServiceProvider' not found

Here's my code:

app.php

'providers' => [    
    ...
    Illuminate\View\ViewServiceProvider::class,
    Collective\Html\HtmlServiceProvider::class,
    ...
],
'aliases' => [
    ...
    'View'      => Illuminate\Support\Facades\View::class,
    'Form'      => Collective\Html\FormFacade::class,
    'Html'      => Collective\Html\HtmlFacade::class
],

composer.json

"require": {
    "php": ">=5.5.9",
    "laravel/framework": "5.1.*",
    "laravelcollective/html": "5.1.*"
},

Any idea what I can do to solve my problem?

via Chebli Mohamed