ffmpeg: 2.7.2 -> 2.8.1 (close #10396)

@vcunat removed 2.7 and 2.6. The global version of 2.6 was unused,
and 2.7 -> 2.8 switch should be without problems by inspecting
http://abi-laboratory.pro/tracker/timeline/ffmpeg/
(only ABI changes and symbol additions)

pkgs/development/libraries/ffmpeg-full/default.nix

···
 
 stdenv.mkDerivation rec {
   name = "ffmpeg-full-${version}";
-  version = "2.7.2";
+  version = "2.8.1";
 
   src = fetchurl {
     url = "https://www.ffmpeg.org/releases/ffmpeg-${version}.tar.bz2";
-    sha256 = "1wlygd0jp34dk4qagi4h9psn4yk8zgyj7zy9lrpm5332mm87bsvw";
+    sha256 = "1qk6g2h993i0wgs9d2p3ahdc5bqr03mp74bk6r1zj6pfinr5mvg2";
   };
 
   patchPhase = ''patchShebangs .'';

pkgs/development/libraries/ffmpeg/2.6.nix

···
-{ callPackage, ... } @ args:
-
-callPackage ./generic.nix (args // rec {
-  version = "${branch}.3";
-  branch = "2.6";
-  sha256 = "1yqc3vm1xrwf866q262qd4nr9d6ifp4gg183pjdc4sl9np0rissr";
-})

pkgs/development/libraries/ffmpeg/2.7.nix

···
-{ callPackage, ... } @ args:
-
-callPackage ./generic.nix (args // rec {
-  version = "${branch}.2";
-  branch = "2.7";
-  sha256 = "1wlygd0jp34dk4qagi4h9psn4yk8zgyj7zy9lrpm5332mm87bsvw";
-})

pkgs/development/libraries/ffmpeg/2.8.nix

···
+{ callPackage, ... } @ args:
+
+callPackage ./generic.nix (args // rec {
+  version = "${branch}.1";
+  branch = "2.8";
+  sha256 = "1qk6g2h993i0wgs9d2p3ahdc5bqr03mp74bk6r1zj6pfinr5mvg2";
+})

pkgs/top-level/all-packages.nix

···
   ffmpeg_0_10 = callPackage ../development/libraries/ffmpeg/0.10.nix { };
   ffmpeg_1_2 = callPackage ../development/libraries/ffmpeg/1.2.nix { };
   ffmpeg_2_2 = callPackage ../development/libraries/ffmpeg/2.2.nix { };
-  ffmpeg_2_6 = callPackage ../development/libraries/ffmpeg/2.6.nix { };
-  ffmpeg_2_7 = callPackage ../development/libraries/ffmpeg/2.7.nix { };
+  ffmpeg_2_8 = callPackage ../development/libraries/ffmpeg/2.8.nix { };
   # Aliases
   ffmpeg_0 = ffmpeg_0_10;
   ffmpeg_1 = ffmpeg_1_2;
-  ffmpeg_2 = ffmpeg_2_7;
+  ffmpeg_2 = ffmpeg_2_8;
   ffmpeg = ffmpeg_2;
 
   ffmpeg-full = callPackage ../development/libraries/ffmpeg-full {