CI: Pin power_assert to v2 as power_assert v3 has dropped support for Ruby < 3.1.

taketo1113 · taketo1113 · commit ba01c8d4cb18 · 2025-11-05T13:53:30.000+09:00
diff --git a/Gemfile b/Gemfile
@@ -5,4 +5,5 @@ gemspec
 gem "rake"
 gem "test-unit"
 gem "test-unit-ruby-core"
+gem "power_assert", "~> 2.0" if RUBY_VERSION < '3.1' # https://github.com/ruby/power_assert/pull/61
 gem "webrick"
