Laravel Valet のインストールで「Your requirements could not be resolved to an installable set of packages.」エラー

# 概要
普段はhomesteadを使用しているのですが、ちょっとした検証アプリなど、サクッと試せる環境が欲しかったので[Valet](https://readouble.com/laravel/5.4/ja/valet.html)環境を構築した。その時に発生したエラーとその対応をメモします。
#エラー内容
```
#実行コマンド
composer global require laravel/valet

#エラー内容
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Conclusion: don't install laravel/valet v2.0.4
    - Conclusion: don't install laravel/valet v2.0.3
    - Conclusion: don't install laravel/valet v2.0.1
    - Conclusion: don't install symfony/process v3.3.4
    - Conclusion: don't install symfony/process v2.6.4|install symfony/process v3.3.4
    - Installation request for laravel/valet ^2.0 -> satisfiable by laravel/valet[v2.0.0, v2.0.1, v2.0.3, v2.0.4].
    - Conclusion: remove symfony/process v2.6.4|install symfony/process v3.3.4
    - laravel/valet v2.0.0 requires symfony/process ~2.7|~3.0 -> satisfiable by symfony/process[v2.7.0, v2.7.1, v2.7.10, v2.7.11, v2.7.12, v2.7.13, v2.7.14, v2.7.15, v2.7.16, v2.7.17, v2.7.18, v2.7.19, v2.7.2, v2.7.20, v2.7.21, v2.7.22, v2.7.23, v2.7.24, v2.7.25, v2.7.26, v2.7.27, v2.7.28, v2.7.29, v2.7.3, v2.7.30, v2.7.31, v2.7.4, v2.7.5, v2.7.6, v2.7.7, v2.7.8, v2.7.9, v2.8.0, v2.8.1, v2.8.10, v2.8.11, v2.8.12, v2.8.13, v2.8.14, v2.8.15, v2.8.16, v2.8.17, v2.8.18, v2.8.19, v2.8.2, v2.8.20, v2.8.21, v2.8.22, v2.8.23, v2.8.24, v2.8.3, v2.8.4, v2.8.5, v2.8.6, v2.8.7, v2.8.8, v2.8.9, v3.0.0, v3.0.1, v3.0.2, v3.0.3, v3.0.4, v3.0.5, v3.0.6, v3.0.7, v3.0.8, v3.0.9, v3.1.0, v3.1.1, v3.1.10, v3.1.2, v3.1.3, v3.1.4, v3.1.5, v3.1.6, v3.1.7, v3.1.8, v3.1.9, v3.2.0, v3.2.1, v3.2.10, v3.2.11, v3.2.2, v3.2.3, v3.2.4, v3.2.5, v3.2.6, v3.2.7, v3.2.8, v3.2.9, v3.3.0, v3.3.1, v3.3.2, v3.3.3, v3.3.4].
    - Can only install one of: symfony/process[v2.7.0, v2.6.4].
    - Can only install one of: symfony/process[v2.7.1, v2.6.4].
    - Can only install one of: symfony/process[v2.7.10, v2.6.4].
    - Can only install one of: symfony/process[v2.7.11, v2.6.4].
    - Can only install one of: symfony/process[v2.7.12, v2.6.4].
    - Can only install one of: symfony/process[v2.7.13, v2.6.4].
    - Can only install one of: symfony/process[v2.7.14, v2.6.4].
    - Can only install one of: symfony/process[v2.7.15, v2.6.4].
    - Can only install one of: symfony/process[v2.7.16, v2.6.4].
    - Can only install one of: symfony/process[v2.7.17, v2.6.4].
    - Can only install one of: symfony/process[v2.7.18, v2.6.4].
    - Can only install one of: symfony/process[v2.7.19, v2.6.4].
    - Can only install one of: symfony/process[v2.7.2, v2.6.4].
    - Can only install one of: symfony/process[v2.7.20, v2.6.4].
    - Can only install one of: symfony/process[v2.7.21, v2.6.4].
    - Can only install one of: symfony/process[v2.7.22, v2.6.4].
    - Can only install one of: symfony/process[v2.7.23, v2.6.4].
    - Can only install one of: symfony/process[v2.7.24, v2.6.4].
    - Can only install one of: symfony/process[v2.7.25, v2.6.4].
    - Can only install one of: symfony/process[v2.7.26, v2.6.4].
    - Can only install one of: symfony/process[v2.7.27, v2.6.4].
    - Can only install one of: symfony/process[v2.7.28, v2.6.4].
    - Can only install one of: symfony/process[v2.7.29, v2.6.4].
    - Can only install one of: symfony/process[v2.7.3, v2.6.4].
    - Can only install one of: symfony/process[v2.7.30, v2.6.4].
    - Can only install one of: symfony/process[v2.7.31, v2.6.4].
    - Can only install one of: symfony/process[v2.7.4, v2.6.4].
    - Can only install one of: symfony/process[v2.7.5, v2.6.4].
    - Can only install one of: symfony/process[v2.7.6, v2.6.4].
    - Can only install one of: symfony/process[v2.7.7, v2.6.4].
    - Can only install one of: symfony/process[v2.7.8, v2.6.4].
    - Can only install one of: symfony/process[v2.7.9, v2.6.4].
    - Can only install one of: symfony/process[v2.8.0, v2.6.4].
    - Can only install one of: symfony/process[v2.8.1, v2.6.4].
    - Can only install one of: symfony/process[v2.8.10, v2.6.4].
    - Can only install one of: symfony/process[v2.8.11, v2.6.4].
    - Can only install one of: symfony/process[v2.8.12, v2.6.4].
    - Can only install one of: symfony/process[v2.8.13, v2.6.4].
    - Can only install one of: symfony/process[v2.8.14, v2.6.4].
    - Can only install one of: symfony/process[v2.8.15, v2.6.4].
    - Can only install one of: symfony/process[v2.8.16, v2.6.4].
    - Can only install one of: symfony/process[v2.8.17, v2.6.4].
    - Can only install one of: symfony/process[v2.8.18, v2.6.4].
    - Can only install one of: symfony/process[v2.8.19, v2.6.4].
    - Can only install one of: symfony/process[v2.8.2, v2.6.4].
    - Can only install one of: symfony/process[v2.8.20, v2.6.4].
    - Can only install one of: symfony/process[v2.8.21, v2.6.4].
    - Can only install one of: symfony/process[v2.8.22, v2.6.4].
    - Can only install one of: symfony/process[v2.8.23, v2.6.4].
    - Can only install one of: symfony/process[v2.8.24, v2.6.4].
    - Can only install one of: symfony/process[v2.8.3, v2.6.4].
    - Can only install one of: symfony/process[v2.8.4, v2.6.4].
    - Can only install one of: symfony/process[v2.8.5, v2.6.4].
    - Can only install one of: symfony/process[v2.8.6, v2.6.4].
    - Can only install one of: symfony/process[v2.8.7, v2.6.4].
    - Can only install one of: symfony/process[v2.8.8, v2.6.4].
    - Can only install one of: symfony/process[v2.8.9, v2.6.4].
    - Can only install one of: symfony/process[v3.0.0, v2.6.4].
    - Can only install one of: symfony/process[v3.0.1, v2.6.4].
    - Can only install one of: symfony/process[v3.0.2, v2.6.4].
    - Can only install one of: symfony/process[v3.0.3, v2.6.4].
    - Can only install one of: symfony/process[v3.0.4, v2.6.4].
    - Can only install one of: symfony/process[v3.0.5, v2.6.4].
    - Can only install one of: symfony/process[v3.0.6, v2.6.4].
    - Can only install one of: symfony/process[v3.0.7, v2.6.4].
    - Can only install one of: symfony/process[v3.0.8, v2.6.4].
    - Can only install one of: symfony/process[v3.0.9, v2.6.4].
    - Can only install one of: symfony/process[v3.1.0, v2.6.4].
    - Can only install one of: symfony/process[v3.1.1, v2.6.4].
    - Can only install one of: symfony/process[v3.1.10, v2.6.4].
    - Can only install one of: symfony/process[v3.1.2, v2.6.4].
    - Can only install one of: symfony/process[v3.1.3, v2.6.4].
    - Can only install one of: symfony/process[v3.1.4, v2.6.4].
    - Can only install one of: symfony/process[v3.1.5, v2.6.4].
    - Can only install one of: symfony/process[v3.1.6, v2.6.4].
    - Can only install one of: symfony/process[v3.1.7, v2.6.4].
    - Can only install one of: symfony/process[v3.1.8, v2.6.4].
    - Can only install one of: symfony/process[v3.1.9, v2.6.4].
    - Can only install one of: symfony/process[v3.2.0, v2.6.4].
    - Can only install one of: symfony/process[v3.2.1, v2.6.4].
    - Can only install one of: symfony/process[v3.2.10, v2.6.4].
    - Can only install one of: symfony/process[v3.2.11, v2.6.4].
    - Can only install one of: symfony/process[v3.2.2, v2.6.4].
    - Can only install one of: symfony/process[v3.2.3, v2.6.4].
    - Can only install one of: symfony/process[v3.2.4, v2.6.4].
    - Can only install one of: symfony/process[v3.2.5, v2.6.4].
    - Can only install one of: symfony/process[v3.2.6, v2.6.4].
    - Can only install one of: symfony/process[v3.2.7, v2.6.4].
    - Can only install one of: symfony/process[v3.2.8, v2.6.4].
    - Can only install one of: symfony/process[v3.2.9, v2.6.4].
    - Can only install one of: symfony/process[v3.3.0, v2.6.4].
    - Can only install one of: symfony/process[v3.3.1, v2.6.4].
    - Can only install one of: symfony/process[v3.3.2, v2.6.4].
    - Can only install one of: symfony/process[v3.3.3, v2.6.4].
    - Installation request for symfony/process (locked at v2.6.4) -> satisfiable by symfony/process[v2.6.4].


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

どうも「symfony/process」のバージョンが古いっぽい。
この記事書いてる時に気づいたが、ちゃんとエラーメッセージにremove して installしろって書いてあるorz

```
Conclusion: remove symfony/process v2.6.4|install symfony/process v3.3.4
```

#対応
```
#symfony/processのアップデート
composer global update symfony/process
```
v3.3.4ではなく、v2.8.24がインストールされましたが、valetはインストールできました。

#補足
どうも古いhomestead環境(v2.0.9)がインストールされてたのが原因っぽい。
削除したら「symfony/process」がアップデートした。

```
composer global remove laravel/homestead

Package operations: 0 installs, 3 updates, 1 removal
  - Removing laravel/homestead (v2.0.9)
  - Updating symfony/debug (v3.0.9 => v3.3.4): Loading from cache
  - Updating symfony/console (v2.8.24 => v3.3.4): Loading from cache
  - Updating symfony/process (v2.8.24 => v3.3.4): Loading from cache
Writing lock file
Generating autoload files
```

以上です。