Update INSTALL.ocaml
Update INSTALL.ocaml

Stable Piqi versions can now be installed using OPAM (package manager for
OCaml).

Motiejus/piqi-deb,Motiejus/piqi-deb,alavrik/piqi,Motiejus/piqi-deb,Motiejus/piqi-deb,alavrik/piqi,alavrik/piqi