fixup! Port to Gstreamer 1.0

`bitrate` isn't a property of `vp8enc`, at least not in gst-plugins-good
1.0.10 (Fedora 18) or 1.2.3 (Fedora 20). Changing it to `target-bitrate`
didn't improve the recorded video quality either. Setting
`{min,max}_quantizer` (as per
https://bugzilla.gnome.org/show_bug.cgi?id=684206) *does* give us decent
quality video, similar to what we had on the GStreamer 0.10 branch.