Set gem metadata

Author: kou

red-flatbuffers.gemspec

@@ -43,4 +43,10 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency("bundler")
   spec.add_development_dependency("rake")
   spec.add_development_dependency("test-unit")
+
+  spec.metadata = {
+    "bug_tracker_uri" => "#{spec.homepage}/issues",
+    "changelog_uri" => "#{spec.homepage}/releases/tag/#{spec.version}",
+    "source_code_uri" => spec.homepage,
+  }
 end