ad-safe-fset is obsolete (replaced by fset)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 issue
 #23).
Merge branch 'ad-safe-fset-obsolete'
Release v1.25.0.