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

By providing a 'changelog_uri' in the metadata of the gemspec a
'Changelog' link will be shown on https://rubygems.org/gems/abbrev
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/abbrev/commit/9643a03ee8

lib/abbrev.gemspec

@@ -19,6 +19,7 @@ Gem::Specification.new do |spec|
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage
+  spec.metadata["changelog_uri"] = spec.homepage + "/releases"
 
   spec.files         = Dir.chdir(File.expand_path('..', __FILE__)) do
     `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }