tremor: rename according to our docs

https://nixos.org/nixpkgs/manual/#sec-package-naming
I forgot to include this commit in the parent merge.

(cherry picked from commit bbe8c1e)

pkgs/development/libraries/tremor/default.nix

@@ -1,7 +1,7 @@
 { stdenv, fetchgit, autoreconfHook, pkgconfig, libogg }:
 
 stdenv.mkDerivation rec {
-  name = "tremor-git-${src.rev}";
+  name = "tremor-unstable-2018-03-16";
 
   src = fetchgit {
     url = https://git.xiph.org/tremor.git;