datakit-client needs a version of lwt recent enought

Otherwise it fails with:

```
  #=== ERROR while compiling datakit-client.0.12.0 ==============================#
  # context              2.2.0~alpha~dev | linux/x86_64 | ocaml-base-compiler.4.04.2 | pinned(https://github.com/moby/datakit/releases/download/0.12.0/datakit-0.12.0.tbz)
  # path                 ~/.opam/4.04/.opam-switch/build/datakit-client.0.12.0
  # command              ~/.opam/opam-init/hooks/sandbox.sh build jbuilder build -p datakit-client -j 127
  # exit-code            1
  # env-file             ~/.opam/log/datakit-client-7-3deb58.env
  # output-file          ~/.opam/log/datakit-client-7-3deb58.out
  ### output ###
  #     ocamldep api/ocaml/datakit_client.depends.ocamldep-output
  #     ocamldep api/ocaml/datakit_client.dependsi.ocamldep-output
  #       ocamlc api/ocaml/datakit_client.{cmi,cmti} (exit 2)
  # (cd _build/default && /home/opam/.opam/4.04/bin/ocamlc.opt -w -40 -g -bin-annot -I /home/opam/.opam/4.04/lib/astring -I /home/opam/.opam/4.04/lib/bytes -I /home/opam/.opam/4.04/lib/cstruct -I /home/opam/.opam/4.04/lib/fmt -I /home/opam/.opam/4.04/lib/lwt -I /home/opam/.opam/4.04/lib/ocaml -I /home/opam/.opam/4.04/lib/ocaml/camlp4 -I /home/opam/.opam/4.04/lib/ocplib-endian -I /home/opam/.opam/4.04/lib/optcomp -I /home/opam/.opam/4.04/lib/sexplib -no-alias-deps -I api/ocaml -o api/ocaml/datakit_client.cmi -c -intf api/ocaml/datakit_client.mli)
  # File "api/ocaml/datakit_client.mli", line 24, characters 7-13:
  # Error: Unbound module Result
```

Author: samoht

packages/datakit-client/datakit-client.0.12.0/opam

@@ -16,7 +16,7 @@ depends: [
   "astring"
   "result"  {>= "1.1" & < "1.5"}
   "fmt"
-  "lwt" {< "5.6"}
+  "lwt" {>= "2.6.0" & < "5.6"}
   "cstruct" {> "2.2.0"}
 ]
 synopsis: "A library to connect to DataKit servers"

packages/datakit-client/datakit-client.0.12.2/opam

@@ -13,7 +13,7 @@ depends: [
   "astring"
   "result"  {>= "1.1" & < "1.5"}
   "fmt"
-  "lwt" {< "5.6"}
+  "lwt" {>= "2.6.0" & < "5.6"}
   "cstruct" {> "2.2.0"}
 ]
 build: ["dune" "build" "-p" name "-j" jobs]

packages/datakit-client/datakit-client.0.12.3/opam

@@ -13,7 +13,7 @@ depends: [
   "astring"
   "result"  {>= "1.1" & < "1.5"}
   "fmt"
-  "lwt" {< "5.6"}
+  "lwt" {>= "2.6.0" & < "5.6"}
   "cstruct" {> "2.2.0"}
 ]
 build: ["dune" "build" "-p" name "-j" jobs]