Update INSTALL to include Num dependency
Update INSTALL to include Num dependency

The num library (including Big_int) was split off the core OCaml distribution starting with 4.06.0:
https://caml.inria.fr/pub/docs/manual-ocaml/libnum.html
s-falke/kittel-koat,s-falke/kittel-koat,s-falke/kittel-koat,s-falke/kittel-koat