Avoid error with latest Rubocop

Author: DirtyF

jekyll-commonmark.gemspec

@@ -23,5 +23,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler", "~> 1.15"
   spec.add_development_dependency "rake", "~> 12.0"
   spec.add_development_dependency "rspec", "~> 3.0"
-  spec.add_development_dependency "rubocop", "~> 0.52"
+  spec.add_development_dependency "rubocop", "~> 0.52.0"
 end