Update dependency

Signed-off-by: Kenji Okimoto <okimoto@clear-code.com>

Author: okkez

fluent-plugin-grok-parser.gemspec

@@ -19,5 +19,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"
   spec.add_development_dependency "test-unit", ">=3.1.5"
-  spec.add_runtime_dependency "fluentd", ">=0.14.6"
+  spec.add_runtime_dependency "fluentd", ">=0.14.6", "< 2"
 end