update gemspec in template
diff --git a/templates/grpc.gemspec.template b/templates/grpc.gemspec.template
index ce775ff..9a41356 100644
--- a/templates/grpc.gemspec.template
+++ b/templates/grpc.gemspec.template
@@ -31,6 +31,7 @@
 
     s.add_dependency 'google-protobuf', '~> 3.0.0.alpha.5.0.3'
     s.add_dependency 'googleauth',      '~> 0.5.1'
+    s.add_dependency 'concurrent-ruby'
 
     s.add_development_dependency 'bundler',            '~> 1.9'
     s.add_development_dependency 'facter',             '~> 2.4'