emacs.scm: Suggest add emacs-next or emacs-snapshot

[tumashu]

Suggest add emacs-next or emacs-snapshot and let guile-emacs inherit it, The 
reason is that
user can use it build emacs from emacs.git conveniently， for example:

    guix build emacs-snapshot --with-source=./emacs-snapshot-20151111.tar.gz