Update dotenv requirement from ~> 2.7.5 to ~> 2.8.1

Updates the requirements on [dotenv](https://github.com/bkeepers/dotenv) to permit the latest version.
- [Release notes](https://github.com/bkeepers/dotenv/releases)
- [Changelog](https://github.com/bkeepers/dotenv/blob/master/Changelog.md)
- [Commits](bkeepers/dotenv@v2.7.5...v2.8.1)

---
updated-dependencies:
- dependency-name: dotenv
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

breathe.gemspec

@@ -29,7 +29,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rake", ">= 12.3.3"
   spec.add_development_dependency "rspec", "~> 3.0"
   spec.add_development_dependency "standard", "~> 0.1.5"
-  spec.add_development_dependency "dotenv", "~> 2.7.5"
+  spec.add_development_dependency "dotenv", "~> 2.8.1"
   spec.add_development_dependency "pry", "~> 0.14.1"
   spec.add_development_dependency "webmock", "~> 3.14.0"