require RuboCop only on recent Rubies

in Gemfile, because gemspec cannot have a RUBY_VERSION condition

Author: mvidner

Gemfile

@@ -4,6 +4,10 @@
 source "https://rubygems.org"
 gemspec
 
+gem "rubocop", "~> 1.68.0" if RUBY_VERSION >= "2.7"
+# newer versions have a noise deprecation warning
+gem "rubocop-ast", "~> 1.36.0" if RUBY_VERSION >= "2.7"
+
 group :test do
   # Optional dependency, we do want to test with it
   gem "nokogiri"

ruby-dbus.gemspec

@@ -34,9 +34,6 @@ GEMSPEC = Gem::Specification.new do |s|
   s.add_development_dependency "packaging_rake_tasks"
   s.add_development_dependency "rake"
   s.add_development_dependency "rspec", "~> 3"
-  s.add_development_dependency "rubocop", "~> 1.68.0"
-  # newer versions have a noise deprecation warning
-  s.add_development_dependency "rubocop-ast", "~> 1.36.0"
   s.add_development_dependency "simplecov"
   s.add_development_dependency "simplecov-lcov"
 end