【Ruby】Rails/Sinatraを使わない場合のrbファイル実行時、i18nに`en is not a valid locale (I18n::InvalidLocale)'怒られる

# 背景
## 記事を書いた理由
公式サイトを見ればすぐに分かる何の事はない内容ですが、
ブログだったりQiitaのような、
わかりやすさ重視の非一次情報はなかなか見つからなかったので。
## 状況
[メタプログラミングRuby](https://www.oreilly.co.jp/books/9784873117430/)の**Monetizeの例**の項を実行していた時。
このような表示に。

```terminal
ruby $ ruby 202010270014.rb
[WARNING] The default rounding mode will change from `ROUND_HALF_EVEN` to `ROUND_HALF_UP` in the next major release. Set it explicitly using `Money.rounding_mode=` to avoid potential problems.
[DEPRECATION] You are using the default localization behaviour that will change in the next major release. Find out more - https://github.com/RubyMoney/money#deprecation
Traceback (most recent call last):
  10: from 202010270014.rb:5:in `<main>'
   9: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/money-6.13.8/lib/money/money.rb:600:in `format'
   8: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/money-6.13.8/lib/money/money/formatter.rb:232:in `to_s'
   7: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/money-6.13.8/lib/money/money/formatter.rb:257:in `format_number'
   6: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/money-6.13.8/lib/money/money/formatter.rb:333:in `format_whole_part'
   5: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/money-6.13.8/lib/money/money/formatter.rb:238:in `thousands_separator'
   4: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/money-6.13.8/lib/money/money/formatter.rb:366:in `lookup'
   3: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/money-6.13.8/lib/money/locale_backend/legacy.rb:15:in `lookup'
   2: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/money-6.13.8/lib/money/locale_backend/i18n.rb:19:in `lookup'
   1: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/i18n-1.6.0/lib/i18n.rb:182:in `translate'
/Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/i18n-1.6.0/lib/i18n.rb:326:in `enforce_available_locales!': :en is not a valid locale (I18n::InvalidLocale)
```

# ソースコード

## エラー発生時
```ruby:error.rb
require "monetize"
bargain_price = Monetize.from_numeric(99, "USD")
p bargain_price.format
```

## エラー解消時
```ruby:success.rb
require "monetize"
I18n.load_path << Dir[File.expand_path("config/locales") + "/*.yml"]
I18n.default_locale = :en # (note that `en` is already the default!)
bargain_price = Monetize.from_numeric(99, "USD")
p bargain_price.format
```
[公式リポジトリ](https://github.com/ruby-i18n/i18n)のREADMEより2,3行目を拝借。
上記のソースの書き換え+config/locales/en.ymlにymlファイルを設置。
その中身は公式リポジトリのREADMEそのままで以下。

```en.yml
en:
  test: "This is a test"
```

### 実行時表示

```terminal
ruby $ ruby 202010270014.rb
[WARNING] The default rounding mode will change from `ROUND_HALF_EVEN` to `ROUND_HALF_UP` in the next major release. Set it explicitly using `Money.rounding_mode=` to avoid potential problems.
[DEPRECATION] You are using the default localization behaviour that will change in the next major release. Find out more - https://github.com/RubyMoney/money#deprecation
[DEPRECATION] You are using the default localization behaviour that will change in the next major release. Find out more - https://github.com/RubyMoney/money#deprecation
"$99.00"
```

WARNINGなんかは出ていますが、一応動作しました。


# 参考文献/サイト
[メタプログラミングRuby](https://www.oreilly.co.jp/books/9784873117430/)
[i18n公式リポジトリ](https://github.com/ruby-i18n/i18n)