fix installation error

Author: bobzhang

.npmignore

@@ -22,6 +22,9 @@ jscomp/**/*
 !jscomp/stubs/ext_basic_hash_stubs.c
 ocaml/
 lib/4.06.1/unstable
+lib/4.06.1/*.d
+lib/4.06.1/bsdep*
+lib/4.06.1/bspp.ml*
 lib/4.06.1/*.cm*
 .circleci/
 native/

scripts/prebuilt.js

@@ -49,7 +49,9 @@ function install() {
   var others_dir = path.join(jscomp_dir, "others");
   var ocaml_dir = path.join(lib_dir, "ocaml");
   var stdlib_dir = path.join(jscomp_dir, "stdlib-406");
-
+  if(!fs.existsSync(ocaml_dir)){
+    fs.mkdirSync(ocaml_dir)
+  }
   // sync up with cmij_main.ml
   installDirBy(runtime_dir, ocaml_dir, function(file) {
     var y = path.parse(file);