Laravel5.6にPassportを導入しようとしたらエラーがでて困った

下記のコマンドでPassportを導入しようとしたところ、

```terminal
$ composer require laravel/passport
```

下記のようなエラーが出ました。

```
Using version ^6.0 for laravel/passport
./composer.json has been updated
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
    - Conclusion: don't install laravel/passport v6.0.6
    - Conclusion: don't install laravel/passport v6.0.5
    - Conclusion: don't install laravel/passport v6.0.4
    - Conclusion: don't install laravel/passport v6.0.3
    - Conclusion: don't install laravel/passport v6.0.2
    - Conclusion: don't install laravel/passport v6.0.1
    - Conclusion: don't install laravel/passport v6.0.0
    - Conclusion: don't install laravel/passport 6.0.x-dev
    - Conclusion: remove paragonie/random_compat v9.99.99
    - Installation request for laravel/passport ^6.0 -> satisfiable by laravel/passport[6.0.x-dev, v6.0.0, v6.0.1, v6.0.2, v6.0.3, v6.0.4, v6.0.5, v6.0.6].
    - Conclusion: don't install paragonie/random_compat v9.99.99
    - laravel/passport 6.0.x-dev requires league/oauth2-server ^7.0 -> satisfiable by league/oauth2-server[7.0.0, 7.1.0, 7.1.1, 7.2.0].
    - league/oauth2-server 7.1.0 requires defuse/php-encryption ^2.1 -> satisfiable by defuse/php-encryption[v2.1.0, v2.2.0].
    - league/oauth2-server 7.1.1 requires defuse/php-encryption ^2.1 -> satisfiable by defuse/php-encryption[v2.1.0, v2.2.0].
    - league/oauth2-server 7.2.0 requires defuse/php-encryption ^2.1 -> satisfiable by defuse/php-encryption[v2.1.0, v2.2.0].
    - league/oauth2-server 7.0.0 requires paragonie/random_compat ^2.0 -> satisfiable by paragonie/random_compat[v2.0.0, v2.0.1, v2.0.10, v2.0.11, v2.0.12, v2.0.13, v2.0.14, v2.0.15, v2.0.16, v2.0.17, v2.0.2, v2.0.3, v2.0.4, v2.0.5, v2.0.6, v2.0.7, v2.0.8, v2.0.9].
    - defuse/php-encryption v2.1.0 requires paragonie/random_compat ~2.0 -> satisfiable by paragonie/random_compat[v2.0.0, v2.0.1, v2.0.10, v2.0.11, v2.0.12, v2.0.13, v2.0.14, v2.0.15, v2.0.16, v2.0.17, v2.0.2, v2.0.3, v2.0.4, v2.0.5, v2.0.6, v2.0.7, v2.0.8, v2.0.9].
    - defuse/php-encryption v2.2.0 requires paragonie/random_compat ~2.0 -> satisfiable by paragonie/random_compat[v2.0.0, v2.0.1, v2.0.10, v2.0.11, v2.0.12, v2.0.13, v2.0.14, v2.0.15, v2.0.16, v2.0.17, v2.0.2, v2.0.3, v2.0.4, v2.0.5, v2.0.6, v2.0.7, v2.0.8, v2.0.9].
    - Can only install one of: paragonie/random_compat[v2.0.0, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.1, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.10, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.11, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.12, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.13, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.14, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.15, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.16, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.17, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.2, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.3, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.4, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.5, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.6, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.7, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.8, v9.99.99].
    - Can only install one of: paragonie/random_compat[v2.0.9, v9.99.99].
    - Installation request for paragonie/random_compat (locked at v9.99.99) -> satisfiable by paragonie/random_compat[v9.99.99].


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

上記のコマンドを実行する前に、下記を実行の実行が必要でした。

```terminal
$ composer require paragonie/random_compat:2.*
```

これが完了した後に再度「composer require laravel/passport」を実行したら、正常に完了しました！