Just noticed that running `./scripts/release/build.js -v 16.1.1` gives me:

```
 ERROR  ENOTDIR: not a directory, open '/Users/gaearon/p/react/packages/.DS_Store/package.json'

Error: 
    at Object.fs.openSync (fs.js:653:18)
    at fs.readFileSync (fs.js:554:33)
    at readdirSync.filter.dir (/Users/gaearon/p/react/scripts/release/utils.js:31:36)
    at Array.filter (<anonymous>)
    at getPublicPackages (/Users/gaearon/p/react/scripts/release/utils.js:29:36)
```

We should ignore system directories there.
'use strict';
const chalk = require('chalk');
const {dots} = require('cli-spinners');
const {exec} = require('child-process-promise');
const {readdirSync, readFileSync} = require('fs');
const logUpdate = require('log-update');
const {join} = require('path');
const execRead = async (command, options) => {