Update to GraphQL Playground 1.7.8.

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

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

package.json

@@ -25,7 +25,7 @@
     "node": ">=6"
   },
   "dependencies": {
-    "@apollographql/graphql-playground-html": "^1.6.0",
+    "@apollographql/graphql-playground-html": "^1.6.4",
     "apollo-server-core": "file:../apollo-server-core",
     "apollo-server-env": "file:../apollo-server-env",
     "graphql-tools": "^4.0.0"