Improve support of the css-mode of Emacs HEAD

The css-mode came to use the SMIE now.

see: http://git.savannah.gnu.org/cgit/emacs.git/commit/lisp/textmodes/css-mode.el?id=0c89e81a86799e0f2226d5ad59e9582d5aefd82c
Merge pull request #1 from elim/support-smie-ver-css-mode

Improve support of the css-mode of Emacs HEAD