Bump deepseq upper bound to 1.6 (#111)

* Thanks to @andreasabel in #109
* Fixes #109

Author: spl

dlist.cabal

@@ -46,7 +46,7 @@ flag Werror
 library
   build-depends:
                         base >= 4 && < 5,
-                        deepseq >= 1.1 && < 1.5
+                        deepseq >= 1.1 && < 1.6
   exposed-modules:      Data.DList
                         Data.DList.Unsafe
   other-modules:        Data.DList.Internal