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

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

spreadsheet.gemspec

@@ -20,4 +20,5 @@ Gem::Specification.new do |spec|
    spec.add_development_dependency "simplecov"
 
    spec.homepage    = 'https://github.com/zdavatz/spreadsheet/'
+   spec.metadata["changelog_uri"] = spec.homepage + "/blob/master/History.md"
 end