`npm` reports Sequelize vulnerability

Looks like we need to upgrade the version (apologies for not PR'ing that change, wanted to at least file the issue). However perhaps I'm not understanding this correctly, as I'm not really a JS developer.

```sh
$ cd examples-orms/node/sequelize
$ npm install
added 112 packages from 146 contributors and audited 226 packages in 2.523s
found 1 high severity vulnerability
run `npm audit fix` to fix them, or `npm audit` for details

╭───────────────────────────────────────────────────────────────╮
│ │
│ New minor version of npm available! 6.5.0 → 6.9.0 │
│ Changelog: https://github.com/npm/cli/releases/tag/v6.9.0 │
│ Run npm i -g npm to update! │
│ │
╰───────────────────────────────────────────────────────────────╯

$ npm audit
=== npm audit security report ===

# Run npm install sequelize@5.7.4 to resolve 1 vulnerability
SEMVER WARNING: Recommended action is a potentially breaking change
┌───────────────┬──────────────────────────────────────────────────────────────┐
│ High │ NoSQL Injection │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ Package │ sequelize │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ Dependency of │ sequelize │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ Path │ sequelize │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ More info │ https://nodesecurity.io/advisories/820 │
└───────────────┴──────────────────────────────────────────────────────────────┘

found 1 high severity vulnerability in 226 scanned packages
1 vulnerability requires semver-major dependency updates.

$ npm audit fix
up to date in 0.42s
fixed 0 of 1 vulnerability in 226 scanned packages
1 package update for 1 vuln involved breaking changes
(use `npm audit fix --force` to install breaking changes; or refer to `npm audit` for steps to fix these manually)
```

examples-orms/node/sequelize から始め、Sequelize のバージョンについて npm の依存関係メタデータを調べます。npm audit を実行して報告された脆弱性を確認し、適切に依存関係を更新して、audit でその脆弱性が報告されなくなり、なおかつ例が引き続き正常にインストールできることを検証します。

索引モデルが issue の本文から書いたものです。