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.

TODO:

- [x] Get an stb-tester 0.19 package for Ubuntu 14.04 on the ppa