gstreamer: 1.4.x -> 1.6.x (all modules)

(And while at it, gst-vaapi 0.6.0 -> 0.6.1.)

* gst-editing-services grew additional build time dependencies, flex and
  perl.

* gst-libav switched from libav to ffmpeg as "libav" provider, see
  http://gstreamer.freedesktop.org/releases/1.6/.
  Without using ffmpeg, one may hit issues such as this (which I
  initially did):

  (gst-plugin-scanner:19751): GStreamer-WARNING **: Failed to load plugin '/nix/store/0wgpq2yx9wrkp2mh4rn1c7zbiq2bqa2l-gst-libav-1.6.1/lib/gstreamer-1.0/libgstlibav.so':
  /nix/store/0wgpq2yx9wrkp2mh4rn1c7zbiq2bqa2l-gst-libav-1.6.1/lib/gstreamer-1.0/libgstlibav.so: undefined symbol: av_frame_get_sample_rate

pkgs/development/libraries/gstreamer/base/default.nix

@@ -4,7 +4,7 @@
 }:
 
 stdenv.mkDerivation rec {
-  name = "gst-plugins-base-1.4.5";
+  name = "gst-plugins-base-1.6.1";
 
   meta = {
     description = "Base plugins and helper libraries";
@@ -16,7 +16,7 @@ stdenv.mkDerivation rec {
 
   src = fetchurl {
     url = "${meta.homepage}/src/gst-plugins-base/${name}.tar.xz";
-    sha256 = "07ampnfa6p41s0lhia62l9h8bdx3c7vxvdz93pbx64m3wycq3gbp";
+    sha256 = "18sbyjcp281zb3bsqji3pglsdsxi0s6ai7rx90sx8cpflkxdqcwm";
   };
 
   nativeBuildInputs = [