configure.ac: bump required GStreamer to 1.3.1

Bumping the required GStreamer version since the codecanalyzer
has a tight dependency to the patch in
https://bugzilla.gnome.org/show_bug.cgi?id=726028.
This fix is missing in older gstreamer versions and
causing the failure of many mpeg2 elementary stream parsing.
(codecanalyzer is only supporing MPEG-2 streams at the moment)