Bump graphiql to 3.7.0 to add support for multipart/mixed incremental field

[giulio-opal]

I realized that GraphiQL wasn't working correctly when using the new multipart mixed transport handler. This is because support for incremental was only added later on graphql/graphiql#3580

I updated to the latest version, 3.7.0: https://github.com/graphql/graphiql/releases/tag/graphiql%403.7.0

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

coverage: 59.26% (-0.02%) from 59.284%
when pulling ae0b4a4 on giulio-opal:giulio/update-graphiql-multipart-incremental
into 41a7be2 on 99designs:master.

[StevenACoffman]

Thanks!