mkv: lower priority of mkv module to 0

I've had it with this module. There is no support, let alone development, from the matroska folks anymore. I prefer to use ffmpeg which seems to be at least less crash prone and having more of a future. I'm not sure which features we are loosing due to this atm. Likely some meta info, probably ordered chapters and definetly the DVD menu capabilities.

modules/demux/mkv.cpp

@@ -116,7 +116,7 @@ static void Close( vlc_object_t * );
 vlc_module_begin();
     set_shortname( "Matroska" );
     set_description( N_("Matroska stream demuxer" ) );
-    set_capability( "demux", 50 );
+    set_capability( "demux", 0 );
     set_callbacks( Open, Close );
     set_category( CAT_INPUT );
     set_subcategory( SUBCAT_INPUT_DEMUX );