Detect the new way oasis enables dynamic configuration

Dynamic configuration used to be declared with `#require "oasis.dynrun"`
in setup.ml but now is simply with `open OASISDynRun`.
Use OASISFormat as a lower bound on oasis version

Sylvain Le Gall says that one can assume
    "OASISFormat: v" ⇒ "(oasis >= v)"

Fixes https://github.com/ocaml/oasis2opam/issues/10