On vscode mac (mojave) : "Unable to load compilation data: Unable to find _build directory"

Hi,

I have tried lot of things, including multiple re-installations of the plugin and downloading another release of the language server and changing `reason_language_server.location`...

I installed OCaml (v4.06.0) and Merlin (v3.1.0) via OPAM. I installed the `reason-vscode` extension via the marketplace. I made a very simple project with only two files:

`main.ml`:
```ocaml
print_endline "Hello, world!"
```
and `dune`:
```ocaml
(executable
(name main))
```
The project compile fine, dune creates the `_build/` folder and the following `.merlin` file:
```
B _build/default/.main.eobjs
S .
FLG -w @a-4-29-40-41-42-44-45-48-58-59-60-40 -strict-sequence -strict-formats -short-paths -keep-locs
```

But the language server doesn't work, each time I update the source file I get a `Unable to find _build directory`. Am I missing something ?

Here is the (not very useful) debug log :
```
[Error - 4:27:27 PM] Request textDocument/codeAction failed.
Message: Unable to find _build directory
Code: -32603
[Error - 4:27:27 PM] Request textDocument/documentSymbol failed.
Message: Unable to find _build directory
Code: -32603
[Error - 4:27:27 PM] Request textDocument/codeAction failed.
Message: Unable to find _build directory
Code: -32603
```

Reproduce the failure with the two-file project using main.ml, dune, the generated .merlin file, and the _build directory. Start by tracing how reason_language_server.location and the language server discover _build on macOS, then verify code actions and document symbols work after editing main.ml.

Written by the indexing model from the issue text.