Bump base bound to allow 4.20

haskeline.cabal

@@ -63,7 +63,7 @@ Library
     -- We require ghc>=7.4.1 (base>=4.5) to use the base library encodings, even
     -- though it was implemented in earlier releases, due to GHC bug #5436 which
     -- wasn't fixed until 7.4.1
-    Build-depends: base >=4.9 && < 4.20, containers>=0.4 && < 0.8,
+    Build-depends: base >=4.9 && < 4.21, containers>=0.4 && < 0.8,
                    directory>=1.1 && < 1.4, bytestring>=0.9 && < 0.13,
                    filepath >= 1.2 && < 1.6, transformers >= 0.2 && < 0.7,
                    process >= 1.0 && < 1.7, stm >= 2.4 && < 2.6,