fixup! Port to Gstreamer 1.0

(I haven't tested this but it's the same change that was made in an
earlier fixup to .travis.yml (which is already squashed into the "Port
to Gstreamer 1.0" commit.), so presumably it's necessary.)
fixup! Port to Gstreamer 1.0

(Presumably this was a mistake; or have you tested that the backslash
isn't necessary on GStreamer 1.0? With this fixup the documentation is
now consistent with extra/vm/README.rst.)