Add changelog to gemspec

For a nice “Changelog” link in the https://rubygems.org/gems/database_cleaner sidebar.

database_cleaner.gemspec

@@ -11,6 +11,8 @@ Gem::Specification.new do |spec|
   spec.homepage    = "https://github.com/DatabaseCleaner/database_cleaner"
   spec.license     = "MIT"
 
+  spec.metadata["changelog_uri"] = "https://github.com/DatabaseCleaner/database_cleaner/blob/master/History.rdoc"
+
   spec.files       = ["lib/database_cleaner.rb"]
   spec.require_paths = ["lib"]