Restrict 'prettier' version to unbreak CI
Restrict 'prettier' version to unbreak CI

Summary:
**what is the change?:**
We have verified that upgrading to Prettier v1.9.0 or v1.9.1 both cause
a Segmentation Fault when running `eslint`. This happens in CI, and I
was able to verify it locally.

**why make this change?:**
We want to use Prettier in CI and not get segmentation faults.

**test plan:**
Locally I did -
`rm -rf node_modules && yarn install && yarn lint` and got no segfault.
Pushing this to github and waiting for CI to pass will further verify.

**issue:**
fixes issue #1562
Closes https://github.com/facebook/draft-js/pull/1567

Differential Revision: D6534157

fbshipit-source-id: 6c3016a7d1fc69151ee34ce73355c3163cc518b8

facebook/draft-js