Update ruby-enum requirement from ~> 0.4 to >= 0.4, < 2.0

Updates the requirements on [ruby-enum](https://github.com/dblock/ruby-enum) to permit the latest version.
- [Changelog](https://github.com/dblock/ruby-enum/blob/master/CHANGELOG.md)
- [Commits](dblock/ruby-enum@v0.4.0...v1.0.0)

---
updated-dependencies:
- dependency-name: ruby-enum
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

mathematical.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib", "ext"]
   spec.extensions    = ["ext/mathematical/extconf.rb"]
 
-  spec.add_dependency("ruby-enum", "~> 0.4")
+  spec.add_dependency("ruby-enum", ">= 0.4", "< 2.0")
 
   spec.add_development_dependency("math-to-itex", "~> 0.3")
   spec.add_development_dependency("minitest", "~> 5.6")