Fix for Emacs 24.4

iswitchb is obsoleted from Emacs 24.4 and I think iswitchb should not be
loaded as default. Because `iswitchb-read-buffer` should be used only
if iswitchb is loaded(`iswitchb-read-buffer` is called when (featurep 'iswitchb)
returns true).
Merge branch 'fix-newer-emacs' into fixed