Updating laravel to laravel 5.5 from laravel 5.4

I'm trying update a laravel from 5.4 version to 5.5. I have done everything with instruction from laravel guide: https://laravel.com/docs/master/upgrade

When I'm trying use command:

composer update

the result is:

Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - The requested package laravel/framework 5.5.* is satisfiable by laravel/framework[5.5.x-dev] but these conflict with your requirements or minimum-stability.

Below I show composer.json file:

{
    "name": "laravel/laravel",
    "description": "The Laravel Framework.",
    "keywords": ["framework", "laravel"],
    "license": "MIT",
    "type": "project",
    "require": {
        "php": ">=5.6.4",
        "doctrine/dbal": "^2.5",
        "intervention/image": "^2.3",
        "laravel/framework": "5.5.*",
        "laravel/tinker": "~1.0",
        "laravelcollective/html": "^5.4.0",
        "unisharp/laravel-filemanager": "^1.7"
    },
    "require-dev": {
        "fzaninotto/faker": "~1.4",
        "mockery/mockery": "0.9.*",
        "phpunit/phpunit": "~6.0"
    },
    "autoload": {
        "classmap": [
            "database"
        ],
        "psr-4": {
            "artSite\\": "app/"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "Tests\\": "tests/"
        }
    },
    "scripts": {
        "post-root-package-install": [
            "php -r \"file_exists('.env') || copy('.env.example', '.env');\""
        ],
        "post-create-project-cmd": [
            "php artisan key:generate"
        ],
        "post-install-cmd": [
            "Illuminate\\Foundation\\ComposerScripts::postInstall",
            "php artisan optimize"
        ],
        "post-update-cmd": [
            "Illuminate\\Foundation\\ComposerScripts::postUpdate",
            "php artisan optimize"
        ]
    },
    "config": {
        "preferred-install": "dist",
        "sort-packages": true
    }
}

I would be greateful for help. Best regards