Missing gstreamer plugins on CI

[jdm]

  │ (servo:27338): GStreamer-WARNING **: External plugin loader failed. This most likely means that the plugin loader helper binary was not found or could not be run. You might need to set the GST_PLUGIN_SCANNER environment variable if your setup is unusual. This should normally not be required though.
  │ 0:00:00.028966467 27338 0x7f2265531780 WARN      GST_PLUGIN_LOADING gstplugin.c:792:_priv_gst_plugin_load_file_for_registry: module_open failed: libmms.so.0: cannot open shared object file: No such file or directory
  │ 
  │ (servo:27338): GStreamer-WARNING **: Failed to load plugin '/home/servo/gst/lib/gstreamer-1.0/libgstmms.so': libmms.so.0: cannot open shared object file: No such file or directory
  │ 0:00:00.029185843 27338 0x7f2265531780 WARN      GST_PLUGIN_LOADING gstplugin.c:792:_priv_gst_plugin_load_file_for_registry: module_open failed: libfaad.so.2: cannot open shared object file: No such file or directory
  │ 
  │ (servo:27338): GStreamer-WARNING **: Failed to load plugin '/home/servo/gst/lib/gstreamer-1.0/libgstfaad.so': libfaad.so.2: cannot open shared object file: No such file or directory
  │ 0:00:00.029414027 27338 0x7f2265531780 WARN      GST_PLUGIN_LOADING gstplugin.c:792:_priv_gst_plugin_load_file_for_registry: module_open failed: libcairo-gobject.so.2: cannot open shared object file: No such file or directory
  │ 
  │ (servo:27338): GStreamer-WARNING **: Failed to load plugin '/home/servo/gst/lib/gstreamer-1.0/libgstcairo.so': libcairo-gobject.so.2: cannot open shared object file: No such file or directory
  │ 0:00:00.029625762 27338 0x7f2265531780 WARN      GST_PLUGIN_LOADING gstplugin.c:792:_priv_gst_plugin_load_file_for_registry: module_open failed: libmpg123.so.0: cannot open shared object file: No such file or directory
  │ 
  │ (servo:27338): GStreamer-WARNING **: Failed to load plugin '/home/servo/gst/lib/gstreamer-1.0/libgstmpg123.so': libmpg123.so.0: cannot open shared object file: No such file or directory

[jdm]

libmms.so.0
libfaad.so.2
libcairo-gobject.so.2
libmpg123.so.0

[ceyusa]

or just disable those elements if we don't have tests using those libraries... though I would enable (including the library) faad (AAC audio format)