Closes #25: In Emacs 24.4, don't turn off font-lock-mode in sly-mrepl

* sly-mrepl.el (sly-mrepl-mode-hook): Only turn off font-lock in Emacs 24.3
(sly-mrepl-mode): Don't turn off `font-lock-mode'
(sly-mrepl--insert-output, sly-mrepl--insert-prompt): Also use a
`font-lock-face' prop.
(sly-mrepl--ensure-prompt-face): New helper.
(sly-mrepl--send-input-sexp, sly-mrepl--insert-prompt): Call
`sly-mrepl--ensure-prompt-face'