Try removing allow-newer (#465)

* Try removing allow-newer

Seems to cause problems for users, e.g. #464

* Update stack84.yaml

Author: ndmitchell

Diff for: stack84.yaml

@@ -20,6 +20,7 @@ extra-deps:
 - parser-combinators-1.2.1
 - haddock-library-1.8.0
 - unordered-containers-0.2.10.0
+- file-embed-0.0.11.2
+- heaps-0.3.6.1
 nix:
   packages: [zlib]
-allow-newer: true