Fix video recording with GStreamer >= 1.1.4

GStreamer >= 1.1.4 requires fully specifying the video-capture-source caps,
otherwise Cheese prints this:

(cheese:22606): cheese-WARNING **: Filter caps do not completely specify the output format: gstcapsfilter.c(348): gst_capsfilter_prepare_buf (): /GstCameraBin:camerabin/GstCapsFilter:videobin-capsfilter:
Output caps are unfixed: video/x-raw, framerate=(fraction)30/1, width=(int)640, height=(int)480, format=(string){ YUY2, YV12, BGR, RGB, I420 }, pixel-aspect-ratio=(fraction)1/1, interlace-mode=(string)progressive

and video-recording is broken. Fixes bug 707391.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>