github 4 needs lower bound on yojson
Fails with
```
=== ERROR while compiling github.4.3.1 =======================================#
context 2.4.0~alpha1~dev | linux/x86_64 | ocaml-base-compiler.4.03.0 | pinned(https://github.com/mirage/ocaml-github/releases/download/4.3.1/github-unix-4.3.1.tbz)
path ~/.opam/4.03/.opam-switch/build/github.4.3.1
command ~/.opam/opam-init/hooks/sandbox.sh build dune build -p github -j 255
exit-code 1
env-file ~/.opam/log/github-7-8bfeb8.env
output-file ~/.opam/log/github-7-8bfeb8.out
ocamlopt lib/.github.objs/native/github_j.{cmx,o} (exit 2)
(cd _build/default && /home/opam/.opam/4.03/bin/ocamlopt.opt -w -40 -principal -strict-sequence -g -safe-string -w A-E-41-42-44-48 -w -27-32 -g -I lib/.github.objs/byte -I lib/.github.objs/native -I /home/opam/.opam/4.03/lib/atdgen -I /home/opam/.opam/4.03/lib/atdgen-runtime -I /home/opam/.opam/4.03/lib/base -I /home/opam/.opam/4.03/lib/base/caml -I /home/opam/.opam/4.03/lib/base/shadow_stdlib -I /home/opam/.opam/4.03/lib/base64 -I /home/opam/.opam/4.03/lib/biniou -I /home/opam/.opam/4.03/lib/bytes -I /home/opam/.opam/4.03/lib/cohttp -I /home/opam/.opam/4.03/lib/cohttp-lwt -I /home/opam/.opam/4.03/lib/easy-format -I /home/opam/.opam/4.03/lib/fieldslib -I /home/opam/.opam/4.03/lib/lwt -I /home/opam/.opam/4.03/lib/re -I /home/opam/.opam/4.03/lib/result -I /home/opam/.opam/4.03/lib/seq -I /home/opam/.opam/4.03/lib/sexplib -I /home/opam/.opam/4.03/lib/sexplib/0 -I /home/opam/.opam/4.03/lib/stringext -I /home/opam/.opam/4.03/lib/uri -I /home/opam/.opam/4.03/lib/yojson -intf-suffix .ml -no-alias-deps -o lib/.github.objs/native/github_j.cmx -c -impl lib/github_j.ml)
File "lib/github_j.ml", line 5064, characters 14-39:
Error: Unbound value Yojson.Safe.write_t
Hint: Did you mean write_int?
ocamlc lib/.github.objs/byte/github_j.{cmo,cmt} (exit 2)
(cd _build/default && /home/opam/.opam/4.03/bin/ocamlc.opt -w -40 -principal -strict-sequence -g -safe-string -w A-E-41-42-44-48 -w -27-32 -g -bin-annot -I lib/.github.objs/byte -I /home/opam/.opam/4.03/lib/atdgen -I /home/opam/.opam/4.03/lib/atdgen-runtime -I /home/opam/.opam/4.03/lib/base -I /home/opam/.opam/4.03/lib/base/caml -I /home/opam/.opam/4.03/lib/base/shadow_stdlib -I /home/opam/.opam/4.03/lib/base64 -I /home/opam/.opam/4.03/lib/biniou -I /home/opam/.opam/4.03/lib/bytes -I /home/opam/.opam/4.03/lib/cohttp -I /home/opam/.opam/4.03/lib/cohttp-lwt -I /home/opam/.opam/4.03/lib/easy-format -I /home/opam/.opam/4.03/lib/fieldslib -I /home/opam/.opam/4.03/lib/lwt -I /home/opam/.opam/4.03/lib/re -I /home/opam/.opam/4.03/lib/result -I /home/opam/.opam/4.03/lib/seq -I /home/opam/.opam/4.03/lib/sexplib -I /home/opam/.opam/4.03/lib/sexplib/0 -I /home/opam/.opam/4.03/lib/stringext -I /home/opam/.opam/4.03/lib/uri -I /home/opam/.opam/4.03/lib/yojson -intf-suffix .ml -no-alias-deps -o lib/.github.objs/byte/github_j.cmo -c -impl lib/github_j.ml)
File "lib/github_j.ml", line 5064, characters 14-39:
Error: Unbound value Yojson.Safe.write_t
Hint: Did you mean write_int?
```
Signed-off-by: Marcello Seri <marcello.seri@gmail.com>