emacs: Expand 'guix-load-path'.

Reported by Mathieu Lirzin <mthl@gnu.org> at
<http://lists.gnu.org/archive/html/guix-devel/2015-11/msg00585.html>.

* emacs/guix-backend.el (guix-start-repl): Expand 'guix-load-path' to
  avoid a potentially relative directory name in Guile %load-path.