Restrict fluentd version explicitly

This is officially recommended way but undocumented.

fluent-plugin-groonga.gemspec

@@ -34,7 +34,7 @@ Gem::Specification.new do |spec|
   spec.test_files += Dir.glob("test/**/*")
   spec.require_paths = ["lib"]
 
-  spec.add_runtime_dependency("fluentd")
+  spec.add_runtime_dependency("fluentd", [">= 0.14.0", "< 2"])
   spec.add_runtime_dependency("groonga-client", ">= 0.1.0")
   spec.add_runtime_dependency("groonga-command-parser")