calculon-web 0.6 neds lower bound on yojson

```
=== ERROR while compiling calculon-web.0.6 ===================================#
context 2.4.0~alpha1~dev | linux/x86_64 | ocaml-base-compiler.4.03.0 | pinned(https://github.com/c-cube/calculon/archive/v0.6.tar.gz)
path ~/.opam/4.03/.opam-switch/build/calculon-web.0.6
command ~/.opam/opam-init/hooks/sandbox.sh build dune build -p calculon-web -j 71
exit-code 1
env-file ~/.opam/log/calculon-web-7-377440.env
output-file ~/.opam/log/calculon-web-7-377440.out
ocamlc src/web/.calculon_web.objs/calculon_web__Giphy_j.{cmo,cmt} (exit 2)
(cd _build/default && /home/opam/.opam/4.03/bin/ocamlc.opt -w -40 -g -bin-annot -I src/web/.calculon_web.objs -I src/web/.calculon_web.objs/.private -I /home/opam/.opam/4.03/lib/ISO8601 -I /home/opam/.opam/4.03/lib/atd -I /home/opam/.opam/4.03/lib/atdgen -I /home/opam/.opam/4.03/lib/biniou -I /home/opam/.opam/4.03/lib/bytes -I /home/opam/.opam/4.03/lib/calculon -I /home/opam/.opam/4.03/lib/containers -I /home/opam/.opam/4.03/lib/curly -I /home/opam/.opam/4.03/lib/easy-format -I /home/opam/.opam/4.03/lib/irc-client -I /home/opam/.opam/4.03/lib/irc-client-lwt -I /home/opam/.opam/4.03/lib/irc-client-lwt-ssl -I /home/opam/.opam/4.03/lib/lambdasoup -I /home/opam/.opam/4.03/lib/logs -I /home/opam/.opam/4.03/lib/lwt -I /home/opam/.opam/4.03/lib/lwt_ssl -I /home/opam/.opam/4.03/lib/re -I /home/opam/.opam/4.03/lib/re/perl -I /home/opam/.opam/4.03/lib/re/posix -I /home/opam/.opam/4.03/lib/re/str -I /home/opam/.opam/4.03/lib/result -I /home/opam/.opam/4.03/lib/ssl -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 -open Calculon_web -o src/web/.calculon_web.objs/calculon_web__Giphy_j.cmo -c -impl src/web/Giphy_j.ml)
File "src/web/Giphy_j.ml", line 26, characters 17-42:
Error: Unbound value Yojson.Safe.write_t
Hint: Did you mean write_int?
```

Signed-off-by: Marcello Seri <marcello.seri@gmail.com>