changed portability links from old inria site to /learn/portability.html

[gurleennsidhuu]

Issue Description

Portability links in Release notes still point back to the old caml inria site. They should instead point to http://preview.ocaml.org/learn/portability.html

Fixes #294

Now the release notes point to /learn/portability.html. Only 2 files were pointing to the old site.