Improve declaration for Node's process.versions
Improve declaration for Node's process.versions

Summary:
Take this snippet:

```js
const nodeVersion = Number(
  process.versions.node.split('.')[0] // WARNING HERE
);

if (nodeVersion >= 4) {
  console.log('yay this is node 4 or higher!');
}
```

Currently, flow warns that `process.versions.node` might be `null`. But that would never be the case.

(Although some of the [process.versions properties](https://nodejs.org/api/process.html#process_process_versions) might not exist on every system, it's a safe bet that `node` and `v8` will always exist, so this PR declares those keys explicitly.)
Closes https://github.com/facebook/flow/pull/2434

Differential Revision: D3864006

Pulled By: mroch

fbshipit-source-id: 2e1007360bb833f76f31eba9ea9d797e70344f70

nmote/flow,MichaelDeBoey/flow,nmote/flow,AprilArcus/flow,gabro/flow,gabelevi/flow,claudiopro/flow,MichaelDeBoey/flow,AprilArcus/flow,nmote/flow,ForbesLindesay/flow,mroch/flow,AprilArcus/flow,TiddoLangerak/flow,gabelevi/flow,fletcherw/flow,samwgoldman/flow,nmote/flow,popham/flow,chenglou/flow,fletcherw/flow,mroch/flow,facebook/flow,chenglou/flow,AgentME/flow,gabelevi/flow,JonathanUsername/flow,mroch/flow,nmote/flow,TiddoLangerak/flow,ForbesLindesay/flow,JonathanUsername/flow,AgentME/flow,ylu1317/flow,gabelevi/flow,popham/flow,mroch/flow,gabro/flow,gabelevi/flow,facebook/flow,JonathanUsername/flow,popham/flow,ForbesLindesay/flow,gabro/flow,gabelevi/flow,jamesgpearce/flow,jgrund/flow,MichaelDeBoey/flow,ylu1317/flow,jamesgpearce/flow,mroch/flow,claudiopro/flow,AgentME/flow,chenglou/flow,gabro/flow,nmote/flow,samwgoldman/flow,jamesgpearce/flow,JonathanUsername/flow,facebook/flow,facebook/flow,facebook/flow,gabro/flow,popham/flow,MichaelDeBoey/flow,gabelevi/flow,samwgoldman/flow,jgrund/flow,facebook/flow,claudiopro/flow,claudiopro/flow,samwgoldman/flow,fletcherw/flow,JonathanUsername/flow,fletcherw/flow,ylu1317/flow,ForbesLindesay/flow,jgrund/flow,fletcherw/flow,jamesgpearce/flow,ylu1317/flow,AgentME/flow,AprilArcus/flow,jamesgpearce/flow,samwgoldman/flow,MichaelDeBoey/flow,mroch/flow,TiddoLangerak/flow,samwgoldman/flow,chenglou/flow,jamesgpearce/flow,chenglou/flow,ForbesLindesay/flow,popham/flow,ForbesLindesay/flow,JonathanUsername/flow,samwgoldman/flow,AprilArcus/flow,gabro/flow,jgrund/flow,popham/flow,mroch/flow,JonathanUsername/flow,fletcherw/flow,chenglou/flow,TiddoLangerak/flow,AprilArcus/flow,popham/flow,AgentME/flow,ylu1317/flow,TiddoLangerak/flow,jgrund/flow,nmote/flow,claudiopro/flow,jgrund/flow,facebook/flow,jamesgpearce/flow