In addition to updating the `@apollographql/graphql-playground-html` fork,
which is necessary to avoid bundling `graphql-config` which had been
problematic in serverless environments, this updates the version number
which is served from the CDN.

I've just published `@apollographql/graphql-playground-html@1.6.4`.

Ref: #1746
Ref: apollographql/graphql-playground#2
Ref: apollographql/graphql-playground#1

Thanks for @javlund for kicking off some of this work!

It's actually the version of `graphql-playground-react` which is getting
served by `graphql-playground-html`.