update yard development dep

Author: jspaleta

CHANGELOG.md

@@ -10,6 +10,7 @@ Which is based on [Keep A Changelog](http://keepachangelog.com/)
 
 ### Changes
 - Use TCPSocket logic with timeout for check-zookeeper-mode.rb and metricz-zookepper.rb  
+- Update development_dependency: yard >= 0.9.20
 
 ### Fixed
 - Remove deplicate count option in check-zookeeper-cluster.rb

sensu-plugins-zookeeper.gemspec

@@ -38,5 +38,5 @@ Gem::Specification.new do |s| # rubocop:disable Metrics/BlockLength
   s.add_development_dependency 'redcarpet',                 '~> 3.2'
   s.add_development_dependency 'rspec',                     '~> 3.4'
   s.add_development_dependency 'rubocop',                   '~> 0.51.0'
-  s.add_development_dependency 'yard',                      '~> 0.8'
+  s.add_development_dependency 'yard',                      '~> 0.9.20'
 end