Add —-recursive option to automatically update submodules from git mgems.

tasks/mruby_build_gem.rake

@@ -91,6 +91,7 @@ module MRuby
           end
         else
           options = [params[:options]] || []
+          options << "--recursive"
           options << "--branch \"#{branch}\""
           options << "--depth 1" unless params[:checksum_hash]
           FileUtils.mkdir_p "#{gem_clone_dir}"