ad-safe-fset is obsolete (replaced by fset)

`ad-safe-fset` has been removed from Emacs. Looking at the corresponding patch, it looks like all references to this function now use `fset` directly: http://lists.gnu.org/archive/html/emacs-diffs/2012-11/msg01170.html.

I launched the espuds tests on emacs snapshot with this change to `el-mock` and all tests passed (except the one talked about in pull request ecukes/espuds#23).