dhall-docs:tasty: Restrict hashable

Author: sjakobi

dhall-docs/dhall-docs.cabal

@@ -137,6 +137,12 @@ Test-Suite tasty
         dhall                             ,
         dhall-docs                        ,
         foldl        < 1.5                ,
+
+        -- hashable isn't used directly, but lucid's rendering output depends
+        -- on the Hashable Text instance defined in hashable.
+        -- See https://github.com/chrisdone/lucid/issues/107.
+        hashable     < 1.3.1              ,
+
         path                              ,
         path-io                           ,
         pretty       >= 1.1.1.1           ,