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

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

Author: mark-young-atg

foreman.gemspec

@@ -16,4 +16,6 @@ Gem::Specification.new do |gem|
   gem.executables = "foreman"
   gem.files = Dir["**/*"].select { |d| d =~ %r{^(README|bin/|data/|ext/|lib/|spec/|test/)} }
   gem.files << "man/foreman.1"
+
+  gem.metadata["changelog_uri"] = gem.homepage + "/blob/master/Changelog.md"
 end