Remove reference to spurious "foo" variable

Prevents byte-compilation warning:

skewer-mode.el:562:9:Warning: assignment to free variable `foo'
Eliminate some warnings.
Explicitly require emacs 24

The code uses `lexical-binding`, so although `js2-mode` depends on Emacs 24, it's best if `skewer-mode` also makes the dependency explicit.
Don't rely on css-backward-sexp

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