Enable multiple threads to build rake gem:native

This speeds up building of all gems. rake-compiler by default will
attempt to launch N + 4 jobs, where N is the number of cores on the
system.

Rakefile

@@ -88,7 +88,7 @@ namespace 'gem' do
   end
 
   desc 'build all native gems'
-  task 'native' => CROSS_RUBY_PLATFORMS
+  multitask 'native' => CROSS_RUBY_PLATFORMS
 end
 
 def add_file_to_gem(relative_source_path)