Update Nix base image (#18)

Author: monacoremo

.circleci/config.yml

@@ -2,7 +2,7 @@ version: 2.1
 jobs:
   build:
     docker:
-      - image: monacoremo/nix:2020-01-14-f9c81b5c-circleci
+      - image: monacoremo/nix:2020-01-31-62bbc2ab-circleci
     steps:
       - checkout
       - run:

shell.nix

@@ -3,8 +3,8 @@ let
     let
       pinnedPkgs =
         builtins.fetchTarball {
-          url = "https://github.com/nixos/nixpkgs/archive/f9c81b5c148572c2a78a8c1d2c8d5d40e642b31a.tar.gz";
-          sha256 = "0ff7zhqk7mjgsvqyp4pa9xjvv9cvp3mh0ss9j9mclgzfh9wbwzmf";
+          url = "https://github.com/nixos/nixpkgs/archive/62bbc2abc1f3ae24943a204a4095c20737189656.tar.gz";
+          sha256 = "0cggas9zl19pzc0ikj9zdxvygqrgmj26snnmhhj27d25kh6cksww";
         };
     in
       import pinnedPkgs {};