ppxlib version requirement

[KihongHeo]

Hi,

In your dune-project, metapp depends on (ppxlib (>= 0.16.0)) but in opam there is an additional constraint ppxlib (>= 0.16.0) & <0.18.0.

I am wondering whether this is intended. I am asking because this makes some version conflicts between clangml and ppx_deriving which my project depends on.

Same for metaquot.

Thanks.

[kit-ty-kate]

This should be fixed by #2