Fix GraphQL 16 compatibility

[djhi]

How to test

1. make install build
2. npm pack
3. create a directory next to json-graphql-server and cd into it
4. yarn install ../json-graphql-server/json-graphql-server-3.0.0.tgz
5. yarn json-graphql-server

You should have a running server at http://localhost:3000 that return a graphiql interface when accessed from a browser

[jaa134]

Thank you for this! Any idea when this will be merged and released?

[jaa134]

@djhi The README mentions express-graphql. You might want to update that section!

The json-graphql-server accepts queries in GET and POST. Under the hood, it uses [the express-graphql module](https://github.com/graphql/express-graphql). Please refer to their documentations for details about passing variables, etc.