Fix schema generation in new TypeScript version (#274)

Fixes #268 

I fixed it by dropping the dependency on quicktype entirely, and using its dependency directly. I still don't understand why the version of typescript used in this repository affects what quicktype is doing, but it seems like the issue is in quicktype, not its dependency.

I validated this change was correct by diffing the output of `node scripts/generate-file-format-schema-json.js` with what's currently on http://speedscope.app/file-format-schema.json. There's no difference.

This PR also includes changes to the CI script to ensure that we can catch this before hitting master next time.