Relax gstreamer version requirements.

Use playbin2 "notify::source" signal instead of "signal::source-setup",
which is not available in gstreamer versions prior to 0.10.33.

Signed-off-by: Ivaylo Dimitrov <freemangordon@abv.bg>

Upstream bug https://bugzilla.mozilla.org/show_bug.cgi?id=836243