GraphQL 15 not supported

[Kmaschta]

Is your feature request related to a problem? Please describe.
Hey, I tried your library, and it seems super cool!
Sadly, I can't make it work with GraphQL 15, because the middleware API seems to have changed.

Describe the solution you'd like
Support the version 15 of GraphQL.

Describe alternatives you've considered
I tried to force the use of graphql 15 with a "resolutions": { "graphql": "^15" } but the middleware failed.

Additional context
Thanks for your effort on that one!

[addityasingh]

@Kmaschta Thanks for the finding. can you send a PR to support GraphQL 15? If not, I will try to create a change as soon as I can