Add dependency to rubocop-fluentd

Author: kenhys

fluent-plugin-fluent-package-update-notifier.gemspec

@@ -35,4 +35,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "test-unit", "~> 3.6.7"
   spec.add_development_dependency "test-unit-rr", "~> 1.0.5"
   spec.add_runtime_dependency "fluentd", [">= 0.14.10", "< 2"]
+
+  spec.add_development_dependency "rubocop-fluentd", "~> 0.2.0"
 end