[ruby/psych] Provide a 'Changelog' link on rubygems.org/gems/psych

By providing a 'changelog_uri' in the metadata of the gemspec
a 'Changelog' link will be shown on https://rubygems.org/gems/psych
which makes it quick and easy for someone to check on the changes
introduced with a new version.

Details of this functionality can be found on https://guides.rubygems.org/specification-reference/

https://github.com/ruby/psych/commit/28ef10edcf

ext/psych/psych.gemspec

@@ -76,5 +76,5 @@ DESCRIPTION
   end
 
   s.metadata['msys2_mingw_dependencies'] = 'libyaml'
-
+  s.metadata['changelog_uri'] = s.homepage + '/releases'
 end