Depending on merlin 3.0.1

It seems to [require merlin 3.0.1](https://github.com/flycheck/flycheck-ocaml/blob/77f8ddbd9bfc3a11/flycheck-ocaml.el#L12), but [merlin is 3.0](https://github.com/ocaml/merlin/blob//0f64255167b63d8eab606419693ac2ca83d132f0/emacs/merlin.el#L7), at the moment.

Is this on purpose, or just an oversight ?

Thanks!