jupyter/completor/merlin: add missing ppx_deriving_yojson preprocessor
jupyter/completor/merlin: add missing ppx_deriving_yojson preprocessor

Signed-off-by: Marcello Seri <34fd14c67154e888f6587d5c0e7eadd13aaf7ca7@gmail.com>

akabe/ocaml-jupyter,akabe/ocaml-jupyter