Milter 1.0.0 requires OCaml < 4.02.0

OCaml 4.02's new string literal syntax `{x|...|x}` causes a parse failure in [this comment](https://github.com/andrenth/ocaml-milter/blob/1.0.0/lib/milter.mli#L161):

```
   - [{unix|local}:/path/to/file] -- A named pipe.
```

as [shown in the bulk logs](https://github.com/ocaml/opam-bulk-logs/blob/master/ubuntu-trusty/4.02.0/20140930/raw/milter#L149-L153)

/cc @andrenth