set displayName for ApolloContext

[dulmandakh]

This PR sets displayName for ApolloContext so that it'll render ApolloContext.Provider instead of Context.Provider in React Devtools.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[apollo-cla]

@dulmandakh: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[benjamn]

Seems like a good idea! Thanks @dulmandakh.