Update VM scripts in `extra/vm/` to Ubuntu 14.04

The vagrant script specifies an Ubuntu 12.04 base image.
12.04 doesn't have GStreamer 1 packages.
Thanks to Thomas von Eyben for reporting this issue.

I haven't fully tested the resulting VM yet.

The install process has an error that I work around by adding an extra step for the user (see step 4 in the README). I don't know yet if I'll be able to fix this in the script itself.