Replace useless core-util-is module with empty stub.

Loading this package triggers a ReferenceError if Buffer is not globally
defined. However, this package never uses Buffer.isBuffer, and in fact
none of this package's properties are ever used by any other packages in
meteor-node-stubs, so it seems safe to replace it with an empty stub.

map.json

@@ -6,6 +6,7 @@
   "console": "console-browserify",
   "constants": "constants-browserify",
   "crypto": "crypto-browserify",
+  "core-util-is": null,
   "dgram": null,
   "dns": null,
   "domain": "domain-browser",

package.json

@@ -6,7 +6,8 @@
   "main": "index.js",
   "license": "MIT",
   "scripts": {
-    "prepublish": "node scripts/build-deps.js"
+    "prepublish": "node scripts/build-deps.js",
+    "postinstall": "node scripts/rm-core-utils.js"
   },
   "dependencies": {
     "assert": "^1.4.1",

scripts/rm-core-utils.js

@@ -0,0 +1,3 @@
+require("rimraf").sync(require("path").join(
+  __dirname, "..", "node_modules", "**", "core-util-is"
+));