Add elpa dependency spec for Emacs 24.1\

This should ensure that Emacs 23 users are not offered this version of
ido-ubiquitous, since it won't work anyway on those versions.
Don't allow overrides to be defined for "nil"

I don't know if anything weird would happen if someone did that, but nil
should never be a valid function.
Hack around a defadvice bug that causes a load error

Hopefully addresses #18.
Version 2.0.7