Update rubocop-rspec to version 3.8.0

Author: depfu[bot]

in_threads.gemspec

@@ -24,6 +24,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rspec-retry', '~> 0.3'
   if RUBY_VERSION >= '2.5'
     s.add_development_dependency 'rubocop', '~> 1.22', '!= 1.22.2'
-    s.add_development_dependency 'rubocop-rspec', '~> 2.0'
+    s.add_development_dependency 'rubocop-rspec', '~> 3.8'
   end
 end