opam2web uses bin-annot, so constrain ocaml-version
constrain dns to ocaml4+
constraint chamo to ocaml4+

```
File "ed_odoc.ml", line 199, characters 17-39:
Error: Unbound module Location
```
add constraint on type_conv on ocamlfind
add missing constraint on pipebang
Merge pull request #407 from mirage/master

more constraints for jenkins