hax to make jest fast

Turns out jest is _incredibly_ slow at resolving require paths like `require('fbjs/lib/foo')`. Like several milliseconds per require. Really adds up when all our files require `invariant` and `warning`. Here's a temporary hack to make things fast again.

Test Plan:
```
npm test src/renderers/shared/reconciler/__tests__/ReactCompositeComponent-test.js
```

has a self-proclaimed runtime of ~8 seconds now instead of ~35 seconds.