Update prelude-lisp.el

Use `completion-at-point` since `lisp-complete-symbol` is obsolete as of Emacs 24.4.
Merge pull request #465 from bixuanzju/patch-1

`lisp-complete-symbol` is obsolete
Merge commit '2c2855841c1dea5199faab6e47653955b8f16f72'

* commit '2c2855841c1dea5199faab6e47653955b8f16f72':
  Update prelude-lisp.el