Don't require Ruby/GStreamer

Because it is not portable yet. :<

It only supports only old GStreamer (0.10.0) but old GStreamer isn't
provided by Homebrew.

We can use new GStrmeamer (1.X) by GObject Introspection based
bindings but GObject Introspection isn't work on Windows. :<

[rabbit-shocker:1089]

Reported by znz. Thanks!!!