not compatible with ocaml 4.03.0+

[avsm]

Compilation fails with this on OCaml 4.03.0+ due to changes in the parsetree:

# File "src/json_types/json_types.ml", line 49, characters 17-30:
# Error: The constructor Pstr_type expects 2 argument(s),
#        but is applied here to 1 argument(s)

The new ocaml-migrate-parsetree package may be helpful in making it more independent of the specific OCaml version in use. cc @let-def

[nv-vn]

Thank you for bringing this up. My schedule currently has very little free time, so the earliest I can look into this will be this weekend.