update gemfile

Author: hiraokashi

pb-serializer.gemspec

@@ -40,4 +40,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "sqlite3", "~> 1.4"
   spec.add_development_dependency "simplecov", "~> 0.18.5"
   spec.add_development_dependency "simplecov-cobertura", "~> 1.3"
+  spec.add_development_dependency "concurrent-ruby", "~> 1.3.4"
 end