zmq-lwt is not compatible with lwt 5.7.0
Due to the catch signature change:
```
\#=== ERROR while compiling zmq-lwt.5.1.5 ======================================#
\# context 2.2.0~alpha3~dev | linux/x86_64 | ocaml-base-compiler.5.1.0 | file:///home/opam/opam-repository
\# path ~/.opam/5.1/.opam-switch/build/zmq-lwt.5.1.5
\# command ~/.opam/opam-init/hooks/sandbox.sh build dune build -p zmq-lwt -j 31 @install
\# exit-code 1
\# env-file ~/.opam/log/zmq-lwt-7-be6d63.env
\# output-file ~/.opam/log/zmq-lwt-7-be6d63.out
\### output ###
\# (cd _build/default && /home/opam/.opam/5.1/bin/ocamlc.opt -w -40 -g -bin-annot -I zmq-lwt/src/.zmq_lwt.objs/byte -I /home/opam/.opam/5.1/lib/bytes -I /home/opam/.opam/5.1/lib/lwt -I /home/opam/.opam/5.1/lib/lwt/unix -I /home/opam/.opam/5.1/lib/ocaml/threads -I /home/opam/.opam/5.1/lib/ocaml/unix -I /home/opam/.opam/5.1/lib/ocplib-endian -I /home/opam/.opam/5.1/lib/ocplib-endian/bigstring -I /home/opam/.opam/5.1/lib/stdint -I /home/opam/.opam/5.1/lib/zmq -I /home/opam/.opam/5.1/lib/zmq/deferred -intf-suffix .ml -no-alias-deps -open Zmq_lwt__ -o zmq-lwt/src/.zmq_lwt.objs/byte/zmq_lwt__Deferred.cmo -c -impl zmq-lwt/src/deferred.ml)
\# File "zmq-lwt/src/deferred.ml", line 1:
\# Error: The implementation zmq-lwt/src/deferred.ml
\# does not match the interface zmq-lwt/src/.zmq_lwt.objs/byte/zmq_lwt__Deferred.cmi:
\# ... In module Deferred:
\# Values do not match:
\# val catch : (unit -> unit -> 'a Lwt.t) -> ('a, exn) Lwt_result.t
\# is not included in
\# val catch : (unit -> 'a t) -> ('a, exn) result t
\# The type (unit -> unit -> 'a Lwt.t) -> ('a, exn) Lwt_result.t
\# is not compatible with the type (unit -> 'b t) -> ('b, exn) result t
\# Type unit -> 'a Lwt.t is not compatible with type 'b t = 'b Lwt.t
\# Hint: Did you forget to provide `()' as argument?
\# File "zmq-deferred/src/deferred.ml", line 6, characters 4-51:
\# Expected declaration
\# File "zmq-lwt/src/deferred.ml", line 5, characters 6-11:
\# Actual declaration
\# (cd _build/default && /home/opam/.opam/5.1/bin/ocamlopt.opt -w -40 -g -I zmq-lwt/src/.zmq_lwt.objs/byte -I zmq-lwt/src/.zmq_lwt.objs/native -I /home/opam/.opam/5.1/lib/bytes -I /home/opam/.opam/5.1/lib/lwt -I /home/opam/.opam/5.1/lib/lwt/unix -I /home/opam/.opam/5.1/lib/ocaml/threads -I /home/opam/.opam/5.1/lib/ocaml/unix -I /home/opam/.opam/5.1/lib/ocplib-endian -I /home/opam/.opam/5.1/lib/ocplib-endian/bigstring -I /home/opam/.opam/5.1/lib/stdint -I /home/opam/.opam/5.1/lib/zmq -I /home/opam/.opam/5.1/lib/zmq/deferred -intf-suffix .ml -no-alias-deps -open Zmq_lwt__ -o zmq-lwt/src/.zmq_lwt.objs/native/zmq_lwt__Deferred.cmx -c -impl zmq-lwt/src/deferred.ml)
\# File "zmq-lwt/src/deferred.ml", line 1:
\# Error: The implementation zmq-lwt/src/deferred.ml
\# does not match the interface zmq-lwt/src/.zmq_lwt.objs/byte/zmq_lwt__Deferred.cmi:
\# ... In module Deferred:
\# Values do not match:
\# val catch : (unit -> unit -> 'a Lwt.t) -> ('a, exn) Lwt_result.t
\# is not included in
\# val catch : (unit -> 'a t) -> ('a, exn) result t
\# The type (unit -> unit -> 'a Lwt.t) -> ('a, exn) Lwt_result.t
\# is not compatible with the type (unit -> 'b t) -> ('b, exn) result t
\# Type unit -> 'a Lwt.t is not compatible with type 'b t = 'b Lwt.t
\# Hint: Did you forget to provide `()' as argument?
\# File "zmq-deferred/src/deferred.ml", line 6, characters 4-51:
\# Expected declaration
\# File "zmq-lwt/src/deferred.ml", line 5, characters 6-11:
\# Actual declaration
```
Signed-off-by: Marcello Seri <marcello.seri@gmail.com>