Provide a 'Changelog' link on rubygems.org/gems/jsbundling-rails (#189)

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

jsbundling-rails.gemspec

@@ -13,4 +13,6 @@ Gem::Specification.new do |spec|
   spec.files += ["LICENSE", "README.md"]
 
   spec.add_dependency "railties", ">= 6.0.0"
+
+  spec.metadata["changelog_uri"] = spec.homepage + "/releases"
 end