Remove the dune warning on merlin #239

$ make
./scripts/before.sh build
opam exec -- dune build @install
File "dune-project", lines 4-46, characters 0-742:
 4 | (cram enable)
 5 | (name columns)
 6 | (allow_approximate_merlin)
....
44 |    ocamlformat
45 |   )
46 |  )
Warning: This field was deprecated in version 2.8 of the dune language. It is
useless since the Merlin configurations are not ambiguous anymore.