Update to work with the GraphQL PHP DefinitionContainer Revert

[jasonbahl]

In version 0.8.0 of graphql-php, the DefinitionContainer was reverted.

If you run composer update things are broken with the current version of the library.

We'll need to update to follow the pattern post-revert, so that we can continue to stay current with that library.

https://github.com/webonyx/graphql-php/commit/0969073b8abd391952eea3b14a4a8b8b7e24aba5