Update power_assert requirement from 2.0.4 to 2.0.5 (#292)

Updates the requirements on [power_assert](https://github.com/ruby/power_assert) to permit the latest version.
- [Release notes](https://github.com/ruby/power_assert/releases)
- [Commits](ruby/power_assert@v2.0.4...v2.0.5)

---
updated-dependencies:
- dependency-name: power_assert
  dependency-type: direct:production
...

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Gemfile

@@ -17,5 +17,5 @@ group :test do
   gem 'warning', '~> 1.5.0'
 
   gem 'irb', '1.14.2'
-  gem 'power_assert', '2.0.4'
+  gem 'power_assert', '2.0.5'
 end