heapster: fix build

fetchFromGitHub returned a different sha256 for the same revision

pkgs/servers/monitoring/heapster/default.nix

···
     inherit rev;
     owner = "kubernetes";
     repo = "heapster";
-    sha256 = "057z9imgd2gvcbvahja3i26jzgm33dmfaxraakmcr4a2xfhj50hq";
+    sha256 = "1vg83207y7yigydnnhlvzs3s94vx02i56lqgs6a96c6i3mr3ydpb";
   };
 
   preBuild = ''