Impossible d'installer cocur/slugify

Bonjour,

Je suis en train de suivre les vidéos de Tutoriel Symfony par l'exemple et je suis arrivé au moment où on doit installer cocur/slugify.
Mais je rencontre un problème : quand j'utilise la commande "composer require cocur/slugify" l'erreur suivante apparait :

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - cocur/slugify v4.0.0 conflicts with symfony/http-kernel[v4.1.13].
    - cocur/slugify v4.0.0 conflicts with symfony/http-kernel[v4.1.13].
    - cocur/slugify v4.0.0 conflicts with symfony/http-kernel[v4.1.13].
    - Installation request for cocur/slugify ^4.0 -> satisfiable by cocur/slugify[v4.0.0].
    - Installation request for symfony/http-kernel (locked at v4.1.13) -> satisfiable by symfony/http-kernel[v4.1.13].

Installation failed, reverting ./composer.json to its original content.

Je n'arrive pas à comprendre le conflit entre http-kernel et slugify étant donné que dans la vidéo tout se passe bien et que j'ai bien initiliasé mon projet avec la commande suivante : composer create-project symfony/website-skeleton MaSuperAgence 4.1.99

Voici à quoi ressemble mon fichier composer.json :

{
    "name": "symfony/website-skeleton",
    "type": "project",
    "license": "MIT",
    "description": "A skeleton to start a new Symfony website",
    "require": {
        "php": "^7.1.3",
        "ext-ctype": "*",
        "ext-iconv": "*",
        "sensio/framework-extra-bundle": "^5.1",
        "symfony/asset": "*",
        "symfony/console": "*",
        "symfony/dotenv": "*",
        "symfony/expression-language": "*",
        "symfony/flex": "^1.3.1",
        "symfony/form": "*",
        "symfony/framework-bundle": "*",
        "symfony/monolog-bundle": "^3.1",
        "symfony/orm-pack": "*",
        "symfony/process": "*",
        "symfony/security-bundle": "*",
        "symfony/serializer-pack": "*",
        "symfony/swiftmailer-bundle": "^3.1",
        "symfony/translation": "*",
        "symfony/twig-bundle": "*",
        "symfony/validator": "*",
        "symfony/web-link": "*",
        "symfony/yaml": "*"
    },
    "require-dev": {
        "symfony/debug-pack": "*",
        "symfony/maker-bundle": "^1.0",
        "symfony/profiler-pack": "*",
        "symfony/test-pack": "*",
        "symfony/web-server-bundle": "*"
    },
    "config": {
        "preferred-install": {
            "*": "dist"
        },
        "sort-packages": true
    },
    "autoload": {
        "psr-4": {
            "App\\": "src/"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "App\\Tests\\": "tests/"
        }
    },
    "replace": {
        "paragonie/random_compat": "2.*",
        "symfony/polyfill-ctype": "*",
        "symfony/polyfill-iconv": "*",
        "symfony/polyfill-php71": "*",
        "symfony/polyfill-php70": "*",
        "symfony/polyfill-php56": "*"
    },
    "scripts": {
        "auto-scripts": {
            "cache:clear": "symfony-cmd",
            "assets:install %PUBLIC_DIR%": "symfony-cmd"
        },
        "post-install-cmd": [
            "@auto-scripts"
        ],
        "post-update-cmd": [
            "@auto-scripts"
        ]
    },
    "conflict": {
        "symfony/symfony": "*"
    },
    "extra": {
        "symfony": {
            "allow-contrib": false,
            "require": "4.1.*"
        }
    }
}

Merci d'avance pour vos réponses =)