eigen: updating from eigen2 to eigen3

pkgs/development/libraries/eigen/default.nix

···
 {stdenv, fetchurl, cmake}:
 
 let
-  v = "2.0.16";
+  v = "3.1.2";
 in
 stdenv.mkDerivation {
   name = "eigen-${v}";
···
   src = fetchurl {
     url = "http://bitbucket.org/eigen/eigen/get/${v}.tar.bz2";
     name = "eigen-${v}.tar.bz2";
-    sha256 = "1akcb4g5hvc664gfc6sxb6f6jrm55fgks6017wg0smyvmm6k09v0";
+    sha256 = "1hywvbn4a8f96fjn3cvd6nxzh5jvh05s1r263d9vqlgn25dxrzay";
   };
   
   buildNativeInputs = [ cmake ];