Merge pull request #288 from line/renovate/rubocop-1.x

chore(deps): update dependency rubocop to '~> 1.51.0'

Author: zenizh

Gemfile

@@ -4,6 +4,6 @@ gemspec
 
 group :development, :test do
   # ref: http://docs.rubocop.org/en/latest/installation/
-  gem 'rubocop', '~> 1.50.0', require: false
+  gem 'rubocop', '~> 1.51.0', require: false
   gem 'yard', '~> 0.9.20'
 end