Prune NPM metadata fields from packages before re-publishing

[bvaughn]

The latest React release has some weird fields in the package JSON that weren't there in the canary. These fields (all beginning with "_") contain package registry meta data.

It's not ideal that we publish package with these fields still around. Technically, they aren't even valid since NPM rules and the packages JSON spec say that field names beginning with underscores are reserved.

So this PR adds a step that prunes these fields as part of preparing a stable release.

JSON diff before

1a2,25
>   "_from": "react@0.0.0-3e15b1c69",
>   "_id": "react@0.0.0-3e15b1c69",
>   "_inBundle": false,
>   "_integrity": "sha512-jGp8MiBrV1FuT0kLcUlb7zjZiaWAcaQ001qCbUen7SHHtnE3Apdl7d7fHshrEOytgoPWSm2pc7vdz1nICqmIDw==",
>   "_location": "/react",
>   "_phantomChildren": {},
>   "_requested": {
>     "type": "version",
>     "registry": true,
>     "raw": "react@0.0.0-3e15b1c69",
>     "name": "react",
>     "escapedName": "react",
>     "rawSpec": "0.0.0-3e15b1c69",
>     "saveSpec": null,
>     "fetchSpec": "0.0.0-3e15b1c69"
>   },
>   "_requiredBy": [
>     "#USER",
>     "/"
>   ],
>   "_resolved": "https://registry.npmjs.org/react/-/react-0.0.0-3e15b1c69.tgz",
>   "_shasum": "a10d5c3fd611046fd5ab65503e6df0b37db99fc0",
>   "_spec": "react@0.0.0-3e15b1c69",
>   "_where": "/Users/bvaughn/Documents/git/react/build/node_modules",
7c31,34
<   "bugs": "https://github.com/facebook/react/issues",
---
>   "bugs": {
>     "url": "https://github.com/facebook/react/issues"
>   },
>   "bundleDependencies": false,
12,13c39,42
<     "scheduler": "0.0.0-3e15b1c69"
<   },  "description": "React is a JavaScript library for building user interfaces.",
---
>     "scheduler": "^0.13.0"
>   },
>   "deprecated": false,
>   "description": "React is a JavaScript library for building user interfaces.",
32,34c61,66
<   "repository": "facebook/react",
<   "version": "0.0.0-3e15b1c69"
< }
\ No newline at end of file
---
>   "repository": {
>     "type": "git",
>     "url": "git+https://github.com/facebook/react.git"
>   },
>   "version": "16.8.0"
> }

JSON diff after

7c7,10
<   "bugs": "https://github.com/facebook/react/issues",
---
>   "bugs": {
>     "url": "https://github.com/facebook/react/issues"
>   },
>   "bundleDependencies": false,
12,13c15,18
<     "scheduler": "0.0.0-3e15b1c69"
<   },  "description": "React is a JavaScript library for building user interfaces.",
---
>     "scheduler": "^0.13.0"
>   },
>   "deprecated": false,
>   "description": "React is a JavaScript library for building user interfaces.",
32,34c37,42
<   "repository": "facebook/react",
<   "version": "0.0.0-3e15b1c69"
< }
\ No newline at end of file
---
>   "repository": {
>     "type": "git",
>     "url": "git+https://github.com/facebook/react.git"
>   },
>   "version": "16.8.0"
> }