Add changelog_uri to gemspec (#1106)

* Add changelog_uri to gemspec

Supported here: https://guides.rubygems.org/specification-reference/#metadata
Useful for running https://github.com/MaximeD/gem_updater

* Remove wrong comma

debug.gemspec

@@ -14,6 +14,7 @@ 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/tag/v#{spec.version}"
 
   # Specify which files should be added to the gem when it is released.
   # The `git ls-files -z` loads the files in the RubyGem that have been added into git.