Declare drb as a dependency for specs

Failure/Error: require 'drb/drb'

LoadError:
  cannot load such file -- drb/drb
# ./lib/rspec/core/drb.rb:1:in `require'
# ./lib/rspec/core/drb.rb:1:in `<top (required)>'
# ./spec/rspec/core/invocations_spec.rb:1:in `require'
# ./spec/rspec/core/invocations_spec.rb:1:in `<top (required)>'

Gemfile

@@ -103,4 +103,6 @@ end
 
 gem 'contracts', '< 0.16' if RUBY_VERSION < '1.9.0'
 
+gem 'drb' if RUBY_VERSION >= '3.4'
+
 eval File.read('Gemfile-custom') if File.exist?('Gemfile-custom')