remove support for guile 1.4

[Doug Evans]

Hi.

I have a patch about to go in that uses hash-fold.
Guile 1.6.8 and later have it but 1.4 doesn't.
[Presumably 1.6.x have it, but 1.6.8 is what I checked.]
We _could_ provide one for 1.4 and add it to guile.scm, but I'm thinking 
that instead we should drop support for 1.4 (it's 7 years old, and our 
limited resources are best spent elsewhere (IMO)).

If anyone objects, please speak up.
Thanks.