Enable `FlexibleContexts` for S.I.S.I.Attoparsec

As GHC 7.10 will otherwise complain that:

  src/System/IO/Streams/Internal/Attoparsec.hs:100:5:
    Non type-variable argument
      in the constraint: Data.String.IsString [a]
    (Use FlexibleContexts to permit this)
    When checking that ‘ctxMsg’ has the inferred type
      ctxMsg :: forall a. Data.String.IsString [a] => [[a]] -> [a]
    In an equation for ‘parseFromStreamInternal’:
        parseFromStreamInternal parseFunc feedFunc parser is
          = Streams.read is
            >>=
              maybe
                (finish $ parseFunc parser "")
                (\ s
                   -> if S.null s then
                          parseFromStreamInternal parseFunc feedFunc parser is
                      else
                          go $! parseFunc parser s)
          where
              leftover x = when (not $ S.null x) $ Streams.unRead x is
              finish k
                = let ...
                  in
                    case k' of {
                      Fail x _ _ -> ...
                      Partial _ -> ...
                      Done x r -> ... }
              err r = let ... in throwIO $ ParseException (ctxMsg c ++ m)
              ctxMsg [] = ""
              ctxMsg xs = "[parsing " ++ intercalate "/" xs ++ "] "
              ....