Constrain lbfgs to OCaml < 4.02.0.

lbfgs doesn't build on OCaml 4.02.*:

```
File "src/lbfgs.mli", line 109, characters 24-26:
Error: String literal begins here
Command exited with code 2.
``
Merge pull request #3262 from yallop/lbfgs-constraint

Constrain lbfgs to OCaml < 4.02.0.