Allow building with mtl-2.1.

haskeline.cabal

@@ -53,7 +53,7 @@ Library
         Build-depends: base>=3 && <4.1 , containers>=0.1 && < 0.3, directory==1.0.*,
                        bytestring==0.9.*
     }
-    Build-depends:  filepath >= 1.1 && < 1.4, mtl >= 1.1 && < 2.1
+    Build-depends:  filepath >= 1.1 && < 1.4, mtl >= 1.1 && < 2.2
     Extensions:     ForeignFunctionInterface, Rank2Types, FlexibleInstances,
                 TypeSynonymInstances
                 FlexibleContexts, ExistentialQuantification