graphql-playground-react@1.7.8 not found on cdn.jsdelivr.net

Hey guys, thanks for the handy tutorial.

I've found an error running the `Graphql Playground`. The `apollo-server-core@2.2.0-alpha.2` (`apollo-server@2.2.0-alpha.2` dep) is setting the `@apollographql/graphql-playground-react@1.7.8` version to `1.7.8`, which doesn't exist anymore on the `cdn.jsdelivr.net`. Here's the playground html source:

```html

```

Updating `apollo-server` to it's current version `2.5.0` fixes the problem, as it changes the `apollographql/graphql-playground-react` version to `1.7.10`.

I will send you guys a PR to update both server/client dependencies, if its ok.