[Opam] Fix installability of eliom 5 with ocaml 4.03

[Kakadu]

At the moment we have some camlp4 related problem. Maybe fixing just opam recipe will be enough?
Anyway, @Drup , says that there is a fix in master but it should be considered for backporting.

➜  ~ opam install eliom.5.0.0
The following dependencies couldn't be met:
  - eliom -> camlp4 <= 4.02+6
Your request can't be satisfied:
  - camlp4<=4.02+6 is not available because your system doesn't comply with !preinstalled & ocaml-version >= "4.02" & ocaml-version < "4.03".

No solution found, exiting
➜  ~ evalopam 
4.03.0
1.2.2
➜  ~ opam pin
➜  ~ 

[vasilisp]

Fixing the OPAM constraints is not enough. We need a few patches (27c69ac and 291adae), but that's the easy part. The hard part is that Eliom 5.0.0 is tied to JSOO 2.7.0, which does not work with 4.03.0.

In summary, if you need OCaml 4.03.0, use Eliom master :).