Make __VERSION__ optional

If it's not present at build time, then it'll be removed completely -- this seems fair because if you're doing your own build then you likely don't match the release version numbers anyway.

I'm not sure how I feel about this though; not sure if this is more logic than desirable for constants.js. It compiles into:

```
if (true) {
  React.version = "0.5.0-alpha";
}
```