Remapped next(previous)-line to ibuffer-forward(backward)-line

* lisp/ibuffer.el (ibuffer-mode-map): same remap is applied on dired.

In GNU Emacs 25.0.50.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.28)
  of 2015-11-07
Repository revision: 87f49c12573334d1d51048cc69ede7b3c121d4a3
Windowing system distributor 'The X.Org Foundation', version 11.0.11702000
System Description:	Debian GNU/Linux testing (stretch)