Fix hackage-to-nix missing old hackage revisions (#597)

Author: hamishmack

nix-tools/nix-tools-src.json

@@ -1,8 +1,8 @@
 {
   "url": "https://github.com/input-output-hk/nix-tools",
-  "rev": "2d817aa6fc5ac3917400e88f205c934107ad3fe9",
-  "date": "2020-05-11T13:44:42+08:00",
-  "sha256": "16934nj7ky8bf6nj7kb4v5bgdcb4i2bc86gj66hmyca2xz15knsw",
+  "rev": "856f3d5a3873f19099ad6d399a8cb52629a92155",
+  "date": "2020-05-12T20:15:19+12:00",
+  "sha256": "0d9c546wgylyzxwflwvkmn3sg21fkwn7a3dbl1qpms7l5164vjbl",
   "fetchSubmodules": false,
   "deepClone": false,
   "leaveDotGit": false