Fix preload SSR API for React v16.6

[jaydenseric]

React v16.6 has some breaking changes to the way the new context API works, causing errors and warnings when attempting to use the preload SSR API, such as:

Warning: Rendering <Context.Consumer.Consumer> is not supported and will be removed in a future major release. Did you mean to render <Context.Consumer> instead?

Warning: Failed prop type: The prop `graphql` is marked as required in `GraphQLQuery`, but its value is `undefined`.

Error: Cannot read property 'constructor' of undefined
      at new GraphQLQuery (file://lib/components.mjs:57:42)

I suspect react-apollo will encounter issues too: apollographql/react-apollo#2530