Rename extra/{ppa => dependencies}

Shortly I will add scripts for building Fedora RPMs too.
extra/dependencies: Build patched gstreamer1-plugins-good for fedora

With the patches for:

* https://bugzilla.gnome.org/show_bug.cgi?id=725632 (not strictly needed
  now, but will be needed for `stbt camera` in a future release)
* https://bugzilla.gnome.org/show_bug.cgi?id=725860 (to support the
  HDPVR with GStreamer 1.x)

These patches have been merged upstream and should be available in
gstreamer1-plugins-good 1.2.4 or 1.2.5 in the near future. In the
meantime you can use this package, published at
https://copr.fedoraproject.org/coprs/stbt/stb-tester/