Upgrade graphql to ^15.1.0
Hello,

Would you mind releasing a new version with graphql dependency bump to ^15.1.0? Signature for `GraphQLScalarType` [was changed in 15.1.0](https://github.com/graphql/graphql-js/pull/2276) and currently peer dependency declaration results in conflict.

Here are [release notes for 15.1.0](https://github.com/graphql/graphql-js/releases/tag/v15.1.0)

Thank you! 🙇 