Update to latest allowed versions of `fbjs` and `prop-types` (#10626)

**what is the change?:**
Updates to the latest allowed versions for two dependencies.

`yarn upgrade 'fbjs@^0.8.9'`
and
`yarn upgrade 'prop-types@^15.5.8'`

**why make this change?:**
Prepping for the 16.0 RC, we want to have a reproducible build that is
as close as possible to what new users will get when installing React.

**test plan:**
`yarn && yarn build && yarn test`

**issue:**
https://github.com/facebook/react/issues/10623