Provide a 'Changelog' link on rubygems.org/gems/websocket

[mark-young-atg]

By providing a 'changelog_uri' in the metadata of the gemspec a 'Changelog' link will be shown on https://rubygems.org/gems/websocket 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/