Add test-unit gem dependencies

Author: cosmo0920

fluent-plugin-elasticsearch-timestamp-check.gemspec

@@ -16,4 +16,5 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency "fluentd", [">= 0.14.0", "< 2"]
   spec.add_development_dependency "rake", "~> 11.0"
   spec.add_development_dependency "bundler", "~> 1.3"
+  spec.add_development_dependency "test-unit", "~> 3.2"
 end