Your requirements could not be resolved to an installable set of packages. Laravel 5.6?

问题

Here is the screenshot of the CMD

Error message in CMD:

Problem 1
- Conclusion: don't install laravel/framework v5.6.29
- Conclusion: don't install laravel/framework v5.6.28
- Conclusion: don't install laravel/framework v5.6.27
- Conclusion: don't install laravel/framework v5.6.26
- Conclusion: don't install laravel/framework v5.6.25
- Conclusion: don't install laravel/framework v5.6.24
- Conclusion: don't install laravel/framework v5.6.23
- Conclusion: don't install laravel/framework v5.6.22
- Conclusion: don't install laravel/framework v5.6.21
- Conclusion: don't install laravel/framework v5.6.20
- Conclusion: don't install laravel/framework v5.6.19
- Conclusion: don't install laravel/framework v5.6.18
- Conclusion: don't install laravel/framework v5.6.17
- Conclusion: don't install laravel/framework v5.6.16
- Conclusion: don't install laravel/framework v5.6.15
- Conclusion: don't install laravel/framework v5.6.14
- Conclusion: don't install laravel/framework v5.6.13
- Conclusion: don't install laravel/framework v5.6.12
- Conclusion: don't install laravel/framework v5.6.11
- Conclusion: don't install laravel/framework v5.6.10
- Conclusion: don't install laravel/framework v5.6.9
- Conclusion: don't install laravel/framework v5.6.8
- Conclusion: don't install laravel/framework v5.6.7
- Conclusion: don't install laravel/framework v5.6.6
- Conclusion: don't install laravel/framework v5.6.5
- Conclusion: don't install laravel/framework v5.6.4
- Conclusion: don't install laravel/framework v5.6.3
- Conclusion: don't install laravel/framework v5.6.2
- Conclusion: don't install laravel/framework v5.6.1
- Conclusion: don't install symfony/process v4.1.3
- Installation request for laravel/framework 5.6.* -> satisfiable by laravel/framework[v5.6.0, v5.6.1, v5.6.10, v5.6.11, v5.6.12, v5.6.13, v5.6.14, v5.6.15, v5.6.16, v5.6.17, v5.6.18, v5.6.19, v5.6.2, v5.6.20, v5.6.21, v5.6.22, v5.6.23, v5.6.24, v5.6.25, v5.6.26, v5.6.27, v5.6.28, v5.6.29, v5.6.3, v5.6.4, v5.6.5, v5.6.6, v5.6.7, v5.6.8, v5.6.9]. - Conclusion: don't install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.1|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.2|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.3|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.4|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.5|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.6|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.7|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.8|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.9|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.10|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.11|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.12|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.13|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.0.14|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.1.0|install phpspec/phpspec 2.5.8
- Conclusion: don't install symfony/process v4.1.1|install phpspec/phpspec 2.5.8
- laravel/framework v5.6.0 requires symfony/process ~4.0 -> satisfiable by symfony/process[v4.0.0, v4.0.1, v4.0.10, v4.0.11,
v4.0.12, v4.0.13, v4.0.14, v4.0.2, v4.0.3, v4.0.4, v4.0.5, v4.0.6, v4.0.7, v4.0.8, v4.0.9, v4.1.0, v4.1.1, v4.1.2, v4.1.3]. - Conclusion: don't install symfony/process v4.1.2|install phpspec/phpspec 2.5.8 - Installation request for phpspec/phpspec ~2.1 -> satisfiable by phpspec/phpspec[2.1.0, 2.1.1, 2.2.0, 2.2.1, 2.3.0, 2.4.0, 2.4.1, 2.5.0, 2.5.1, 2.5.2, 2.5.3, 2.5.4, 2.5.5, 2.5.6, 2.5.7, 2.5.8].

My composure:

{
    "name": "laravel/laravel",
    "description": "The Laravel Framework.",
    "keywords": ["framework", "laravel"],
    "license": "MIT",
    "type": "project",
    "require": {
        "laravel/framework": "5.6.*"
    },
    "require-dev": {
        "phpunit/phpunit": "~4.0",
        "phpspec/phpspec": "~2.1"
    },
    "autoload": {
        "classmap": [
            "database"
        ],
        "psr-4": {
            "App\\": "app/"
        }
    },
    "autoload-dev": {
        "classmap": [
            "tests/TestCase.php"
        ]
    },
    "scripts": {
        "post-install-cmd": [
            "php artisan clear-compiled",
            "php artisan optimize"
        ],
        "post-update-cmd": [
            "php artisan clear-compiled",
            "php artisan optimize"
        ],
        "post-create-project-cmd": [
            "php -r \"copy('.env.example', '.env');\"",
            "php artisan key:generate"
        ]
    },
    "config": {
        "preferred-install": "dist"
    }
}

How can I update to Laravel 5.6 ?