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>
Merge pull request #9 from freemangordon/embedlite_gstreamer_version

Relax gstreamer version requirements.