Laravel 5.5で Horizon をインストールするときの注意点

# 前提
Laravel 5.5
PHP 7.2

# 起きたこと
Laravel Horizon をインストールしようと
https://laravel.com/docs/5.5/horizon
に記載の通り

```console
composer require laravel/horizon
```

したら派手に怒られてしまった。

```
 Problem 1
    - Conclusion: remove laravel/framework v5.5.45
    - Conclusion: don't install laravel/framework v5.5.45
    - laravel/horizon v3.1.0 requires illuminate/queue ~5.7.0|~5.8.0|~5.9.0 -> satisfiable by illuminate/queue[5.7.17, 5.7.18, 5.7.19, v5.7.0, v5.7.1, v5.7.10, v5.7.11, v5.7.15, v5.7.2, v5.7.20, v5.7.21, v5.7.22, v5.7.23, v5.7.26, v5.7.27, v5.7.28, v5.7.3, v5.7.4, v5.7.5, v5.7.6, v5.7.7, v5.7.8, v5.7.9, v5.8.0, v5.8.2, v5.8.3, v5.8.4, v5.8.8, v5.8.9].
    - laravel/horizon v3.1.1 requires illuminate/queue ~5.7.0|~5.8.0|~5.9.0 -> satisfiable by illuminate/queue[5.7.17, 5.7.18, 5.7.19, v5.7.0, v5.7.1, v5.7.10, v5.7.11, v5.7.15, v5.7.2, v5.7.20, v5.7.21, v5.7.22, v5.7.23, v5.7.26, v5.7.27, v5.7.28, v5.7.3, v5.7.4, v5.7.5, v5.7.6, v5.7.7, v5.7.8, v5.7.9, v5.8.0, v5.8.2, v5.8.3, v5.8.4, v5.8.8, v5.8.9].
    - don't install illuminate/queue 5.7.17|don't install laravel/framework v5.5.45
    - don't install illuminate/queue 5.7.18|don't install laravel/framework v5.5.45
    - don't install illuminate/queue 5.7.19|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.0|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.1|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.10|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.11|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.15|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.2|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.20|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.21|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.22|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.23|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.26|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.27|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.28|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.3|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.4|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.5|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.6|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.7|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.8|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.7.9|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.8.0|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.8.2|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.8.3|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.8.4|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.8.8|don't install laravel/framework v5.5.45
    - don't install illuminate/queue v5.8.9|don't install laravel/framework v5.5.45
    - Installation request for laravel/framework (locked at v5.5.45, required as 5.5.*) -> satisfiable by laravel/framework[v5.5.45].
    - Installation request for laravel/horizon ^3.1 -> satisfiable by laravel/horizon[v3.1.0, v3.1.1].

```

# 原因
Laravel Horizonの最新バージョンは「[v3.1.1](https://packagist.org/packages/laravel/horizon#v3.1.1)」で、Laravel 5.7.0 以上しか対応していないため。

# どうしても使いたい場合は
- Laravelのメジャーバージョンアップ(5.7以上に)
- Laravel5.5でも利用できるHorizonをインストール（古いバージョンを利用するというリスクを踏まえた上でお使いください。）

```console

composer require laravel/horizon:v2.2.2

```