Remove code to add rbenv paths to exec-path

PATH is parsed at startup, so `(add-to-list 'exec-path <...>)' is
unnecessary (if harmless) as long as the required directories are
already in PATH.  Export PATH in an rc file and call Emacs from the
shell by `exec $SHELL -c emacs' to ensure this.
Merge remote-tracking branch 'refs/remotes/origin/master'