Don't use Control.Exception.Extensible in Setup.hs.

This fixes the build on ghc>=7.6.1, which no longer provides the
extensible-exceptions package by default.  (Haskeline's own
dependency on extensible-exceptions was removed in haskeline-0.7.0.0.)

Setup.hs

@@ -14,7 +14,7 @@ import qualified Distribution.InstalledPackageInfo as Installed
 import System.IO
 import System.Exit
 import System.Directory
-import Control.Exception.Extensible
+import Control.Exception
 import Control.Monad(when)
 
 main :: IO ()