Still stay on async-1.x

Because dependent io-event (v1.0.9) can't build on Windows.

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

Author: ashie

fluentd.gemspec

@@ -49,5 +49,6 @@ Gem::Specification.new do |gem|
   gem.add_development_dependency("test-unit", ["~> 3.3"])
   gem.add_development_dependency("test-unit-rr", ["~> 1.0"])
   gem.add_development_dependency("oj", [">= 2.14", "< 4"])
+  gem.add_development_dependency("async", "~> 1.23")
   gem.add_development_dependency("async-http", ">= 0.50.0")
 end