Disable warn-error, as it makes developing on a trunk compiler hard

This is currently blocking the upstream ocaml/ocaml Travis tests from passing on trunk, since we:
* have to use OPAM trunk (since ocp-build doesnt work on 4.02)
* have new errors (deprecation warnings) that shouldn't be fatal

Disabling -warn-error and keeping an eye out for errors during normal development is vastly preferable...