do not use master for revision

This commit changes the revision in use for the `easy-ps` derivation,
moving away from using `master` in favor of a predictable commit SHA.

This avoids that the build will suddenly stop working,because the
underlying repository gets updated.

nix/easy-ps.nix

@@ -5,7 +5,7 @@ import
     pkgs.fetchFromGitHub {
       owner = "justinwoo";
       repo = "easy-purescript-nix";
-      rev = "master";
+      rev = "11d3bd58ce6e32703bf69cec04dc7c38eabe14ba";
       sha256 = "tESal32bcqqdZO+aKnBzc1GoL2mtnaDtj2y7ociCRGA=";
     }
   ) {