Fix dune warning

[samoht]

Otherwise building the project gives:

$ dune build
File "dune-project", line 3, characters 14-17:
3 | (using menhir 2.0)
                  ^^^
Warning: Version 2.0 of the menhir extension is not supported until version
1.4 of the dune language.
Supported versions of this extension in version 1.2 of the dune language:
- 1.0

[hannesm]

LGTM

[samoht]

@hannesm is that ok if I make a point release of charrua with just that fix?

[hannesm]

@samoht sure, I'd propose to update all the opam files in this repository to depend on "dune" {>= "1.4.0"}.

[samoht]

Ok I've add constraints to the dune files and a changelog entry.

[hannesm]

LGTM