Disable -warn-error so that ocamlclean builds with OCaml 4.02.0.

Pre-patch:
```
### stdout ###
# ...[truncated]
# File "index.ml", line 23, characters 18-31:
# Warning 3: deprecated: String.create
# File "index.ml", line 24, characters 13-26:
# Warning 3: deprecated: String.create
# File "index.ml", line 1:
# Error: Some fatal warnings were triggered (2 occurrences)
```
/cc @bvaugon