Build error on macos

[dboris]

After installing ppx_deriving_yojson, yojson was downgraded from 1.7.0 to 1.5.0, and ocsigen-start fails to build with the following error:

#=== ERROR while compiling ocsigen-start.1.7.0 ================================#
# context     2.0.4 | macos/x86_64 | ocaml-base-compiler.4.07.1 | https://opam.ocaml.org#393c2feb
# path        ~/.opam/ocsigen.407/.opam-switch/build/ocsigen-start.1.7.0
# command     ~/.opam/opam-init/hooks/sandbox.sh build make -j4
# exit-code   2
# env-file    ~/.opam/log/ocsigen-start-14204-1872ec.env
# output-file ~/.opam/log/ocsigen-start-14204-1872ec.out
### output ###
# [...]
# eliomc -ppx -c -package lwt_ppx -package js_of_ocaml-ppx.deriving -package calendar -package safepass -package ocsigen-toolkit.server -package yojson -package re.str -I _server/src/ -g src/os_lib.eliomi
# eliomc -ppx -c -package lwt_ppx -package js_of_ocaml-ppx.deriving -package calendar -package safepass -package ocsigen-toolkit.server -package yojson -package re.str -I _server/src/ -g src/os_connect_phone.eliomi
# eliomc -ppx -c -package lwt_ppx -package js_of_ocaml-ppx.deriving -package calendar -package safepass -package ocsigen-toolkit.server -package yojson -package re.str -I _server/src/ -g src/os_date.eliomi
# eliomc -ppx -c -package lwt_ppx -package js_of_ocaml-ppx.deriving -package calendar -package safepass -package ocsigen-toolkit.server -package yojson -package re.str -I _server/src/ -g src/os_email.eliomi
# eliomc -ppx -c -package lwt_ppx -package js_of_ocaml-ppx.deriving -package calendar -package safepass -package ocsigen-toolkit.server -package yojson -package re.str -I _server/src/ -g src/os_fcm_notif.eliomi
# eliomc -ppx -c -package lwt_ppx -package js_of_ocaml-ppx.deriving -package calendar -package safepass -package ocsigen-toolkit.server -package yojson -package re.str -I _server/src/ -g src/os_services.eliomi
# eliomc -ppx -c -package lwt_ppx -package js_of_ocaml-ppx.deriving -package calendar -package safepass -package ocsigen-toolkit.server -package yojson -package re.str -I _server/src/ -g src/os_msg.eliomi
# eliomc -ppx -c -package lwt_ppx -package js_of_ocaml-ppx.deriving -package calendar -package safepass -package ocsigen-toolkit.server -package yojson -package re.str -I _server/src/ -g src/os_handlers.eliomi
# File "src/os_fcm_notif.eliomi", line 99, characters 23-36:
# Error: Unbound type constructor Yojson.Safe.t
# make: *** [_server/src/os_fcm_notif.cmi] Error 2
# make: *** Waiting for unfinished jobs....

[dboris]

Related to ocaml-ppx/ppx_deriving_yojson#94
Pinning ppx_deriving_yojson as suggested there resolves the issue.