Lumen getting satisfiable by illuminate/console error on installing package

in latest version of Lumen I'm trying to install lumen-generator package but i get this error:

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

  Problem 1
    - Installation request for flipbox/lumen-generator ^8.0 -> satisfiable by flipbox/lumen-generator[8.0.0].
    - Conclusion: remove illuminate/console v5.1.41
    - Conclusion: don't install illuminate/console v5.1.41
    - flipbox/lumen-generator 8.0.0 requires illuminate/console ^5.5|^6.0|^7.0|^8.0 -> satisfiable by illuminate/console[5.7.17, 5.7.18, 5.7.19, ... , v7.4.0, v7.5.0, v7.5.1, v7.5.2, v7.6.0, v7.6.1, v7.6.2, v7.7.0, v7.7.1, v7.8.0, v7.8.1, v7.9.0, v7.9.1, v7.9.2, v8.0.0, v8.0.1, v8.0.2, v8.0.3, v8.0.4, v8.1.0, v8.2.0, v8.3.0, v8.4.0, v8.5.0, v8.6.0, v8.7.0, v8.7.1].
    - Can only install one of: illuminate/console[5.7.17, v5.1.41].
    - Can only install one of: illuminate/console[5.7.18, v5.1.41].
    - Can only install one of: illuminate/console[5.7.19, v5.1.41].
    ...
    - Can only install one of: illuminate/console[v8.7.1, v5.1.41].
    - Installation request for illuminate/console (locked at v5.1.41) -> satisfiable by illuminate/console[v5.1.41].

i can't resolve this problem with adding this lines into composer.json

    "minimum-stability": "dev",
    "prefer-stable": true,

composer.json content:

    {
        "name": "laravel/lumen",
        "description": "The Laravel Lumen Framework.",
        "keywords": ["framework", "laravel", "lumen"],
        "license": "MIT",
        "type": "project",
        "require": {
            "php": ">=5.5.9",
            "laravel/lumen-framework": "5.1.*",
            "vlucas/phpdotenv": "~1.0"
        },
        "require-dev": {
            "phpunit/phpunit": "~4.0",
            "fzaninotto/faker": "~1.0"
        },
        "autoload": {
            "psr-4": {
                "App\\": "app/"
            },
            "classmap": [
                "database/"
            ]
        },
        "minimum-stability": "dev",
        "prefer-stable": true,
        "autoload-dev": {
            "classmap": [
                "tests/"
            ]
        },
        "config": {
            "preferred-install": "dist"
        }
    }

is any body can help me resolve this issue? thanks in advance