Don't rely on css-backward-sexp
Don't rely on css-backward-sexp

In recent Emacs snapshots, there's no longer a css-backward-sexp.

emacsmirror/skewer-mode,skeeto/skewer-mode,emacsmirror/skewer-mode,skeeto/skewer-mode,kostafey/skewer-mode,syohex/skewer-mode,kostafey/skewer-mode,syohex/skewer-mode