Add ppx_type_conv lower bound

[rgrinberg]

Previous versions don't interact with ppx_deriving that well.

Best would be to somehow black list this version in opam-repository. But it doesn't hurt adding this here.

@hannesm

[hannesm]

@rgrinberg thanks! if ppx_type_conv v0.9.0 does not work anywhere (does it work anywhere?), I'd be in favour of deleting it from opam-repository!?
as far as I understand @diml, ppx_type_conv v0.9.0 needs a conflict with ppx_deriving.4.2!? once this is added, is this change here needed (well, if cohttp depends on ppx_type_conv, it should have a dependency in opam). I'm not familiar with either ppx_type_conv, ppx_deriving, nor cohttp to judge which change(s) are needed.

[rgrinberg]

if ppx_type_conv v0.9.0 does not work anywhere (does it work anywhere?), I'd be in favour of deleting it from opam-repository!?

It does work for other converters that don't generate modules, since in this situation generating the same code doesn't hurt.

[ghost]

Adding the conflict seems fine. Hopefully it shouldn't make packages uninstallable