when new version detected, show command to update

lib/cocoapods/sources_manager.rb

@@ -206,7 +206,8 @@ module Pod
         end
 
         if config.new_version_message? && cocoapods_update?(versions)
-          UI.puts "\nCocoaPods #{versions['last']} is available.\n".green
+          UI.puts "\nCocoaPods #{versions['last']} is available.\n" \
+            "To update use: [sudo] gem update cocoapods\n".green
         end
       end