install: let preinstall can update package.json

hi, @npm, I created this module [npm-private-cli](https://github.com/yorkie/npm-private-cli) that would update the `dependencies` of my package.json, AND i post this script into my preinstall of scripts in package.json:
```JSON
{
  "scripts": { "preinstall": "npm-private -u yorkie -p xxx" }
}
```

Then the result what I except is not in my shell, `preinstall` script had updated my package.json, but `npm install` always use the old version of the package.json, then I notice that you enable the cache in your `readJson` module/function, so I think the correct or what user expected is `npm install` should use the latest package.json(disabled cache).

Also, I would make a PR for your `read-package-json` module to enable a cache control argument.
:)