Switch Markdown dependency to commonmarker

Author: Yuki Izumi

Gemfile

@@ -4,7 +4,7 @@ gemspec
 gem "posix-spawn", :platforms => :ruby
 gem "redcarpet", :platforms => :ruby
 gem "kramdown", :platforms => :jruby
-gem "RedCloth"
+gem "commonmarker", "~> 0.14.9"
 gem "rdoc", "~>3.6"
 gem "org-ruby", "= 0.9.9"
 gem "creole", "~>0.3.6"