Running `npm run build` fails on OS X

Here's the output I see in my terminal:

```
> reprocessing-example@ build /Users/fhsu/reference/reprocessing-example
> bsb -make-world

[2/2] Building fake_src/sdl_index.mlast.d
[2/2] Building lib.cma
[4/4] Building run_build_script
FAILED: run_build_script
/Users/fhsu/reference/reprocessing-example/node_modules/Tsdl/lib/bs/bytecode/build_script.exe /Users/fhsu/reference/reprocessing-example/node_modules/bs-platform/vendor /Users/fhsu/reference/reprocessing-example/node_modules/bs-platform/vendor/ocaml/lib/ocaml /Users/fhsu/reference/reprocessing-example/node_modules/Tsdl /Users/fhsu/reference/reprocessing-example
Fatal error: cannot load shared library dllunix
Reason: dlopen(/usr/local/lib/ocaml/stublibs/dllunix.so, 138): Symbol not found: _caml_ba_compare
Referenced from: /usr/local/lib/ocaml/stublibs/dllunix.so
Expected in: flat namespace
in /usr/local/lib/ocaml/stublibs/dllunix.so
ninja: error: rebuilding 'build.ninja': subcommand failed
Failure: /Users/fhsu/reference/reprocessing-example/node_modules/bs-platform/lib/ninja.exe
Location: /Users/fhsu/reference/reprocessing-example/node_modules/Tsdl/lib/bs/bytecode
npm ERR! code ELIFECYCLE
npm ERR! errno 2
npm ERR! reprocessing-example@ build: `bsb -make-world`
npm ERR! Exit status 2
npm ERR!
npm ERR! Failed at the reprocessing-example@ build script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR! /Users/fhsu/.npm/_logs/2018-07-02T18_39_59_156Z-debug.log
```