Update bigdecimal requirement from ~> 3.1 to ~> 4.0 (#444)

Updates the requirements on [bigdecimal](https://github.com/ruby/bigdecimal) to permit the latest version.
- [Release notes](https://github.com/ruby/bigdecimal/releases)
- [Changelog](https://github.com/ruby/bigdecimal/blob/master/CHANGES.md)
- [Commits](ruby/bigdecimal@v3.1.0...v4.0.1)

---
updated-dependencies:
- dependency-name: bigdecimal
  dependency-version: 4.0.1
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile

@@ -5,7 +5,7 @@ source "https://rubygems.org"
 gemspec
 
 gem "base64", "~> 0.3.0"
-gem "bigdecimal", "~> 3.1"
+gem "bigdecimal", "~> 4.0"
 gem "coveralls", "~> 0.8", require: false
 gem "csv", "~> 3.2"
 gem "rubocop", "~> 1.75", require: false