servant-server: Bump transformers upper bound

Author: bgamari

servant-server/servant-server.cabal

@@ -62,7 +62,7 @@ library
       , system-filepath    >= 0.4  && < 0.5
       , filepath           >= 1
       , text               >= 1.2  && < 1.3
-      , transformers       >= 0.3  && < 0.5
+      , transformers       >= 0.3  && < 0.6
       , transformers-compat>= 0.4
       , wai                >= 3.0  && < 3.3
       , wai-app-static     >= 3.0  && < 3.2