tutorial: fix for ghc-7.8

doc/tutorial/Server.lhs

@@ -46,6 +46,9 @@ need to have some language extensions and imports:
 
 module Server where
 
+import Prelude ()
+import Prelude.Compat
+
 import Control.Monad.IO.Class
 import Control.Monad.Reader
 import Control.Monad.Trans.Except

doc/tutorial/tutorial.cabal

@@ -22,6 +22,7 @@ library
   -- other-modules:
   -- other-extensions:
   build-depends:       base == 4.*
+                     , base-compat
                      , text
                      , aeson
                      , blaze-html