[ESS] Updated distributions of Emacs + better integration of ess-nuke-trailing-whitespace

[Vincent Goulet]

I updated my Emacs Modified distributions to 25.2 and org 9.0.6:

	macOS: https://vigou3.github.io/emacs-modified-macos/
	Windows: https://vigou3.github.io/emacs-modified-windows/

Of some special interest may be the following change in the configuration:

- In `site-start.el`, `ess-nuke-trailing-whitespace` is now added to
  the `write-contents-functions` hook to avoid bleeding of the
  functionality to other major modes (most notably markdown, where
  trailing whitespace is sometimes useful).

I just recently realized that the former configuration (using the write-files-functions hook) ended up in global activation of the nuke trailing whitespace functionality. I hope this change proves useful.

v.