.emacs: move package.el specefic code into ~/src/github/emacs.d
.emacs_d/lisp/bhj-eval-after-load.el: do not require auto-complete-clang here
bin/Linux/emacs-install-packages: do not use package.el when emacs version too low; use require-package
etc/custom.el: do not try to load yasnippet in case it's emacs23

Signed-off-by: Bao Haojun <baohaojun@gmail.com>