Restore hex and random_bytes to SecureRandom 1.8 (fixes Travis)

The refactor which replaced hex and random_bytes with native code in SecureRandom accidentally left them broken for 1.8 (the [commit which native-ed uuid](https://github.com/jruby/jruby/commit/14f7c4780217f939adc6314201eca909773eb1fb) moved the native extension loading behind a Ruby 1.9-only check).  Restored!

This gets `jruby-1_7` Travis green.