Fix iOS e2e tests: bump react-native-babel-preset to ^5 (#19625)

Summary:
We opt in to version ^5 of the React Native Babel Preset, as required after the bump to Babel 7. This fixes the Objective-C end-to-end test failure in master. (Fixes #19538)

See https://github.com/facebook/react-native/commit/34bd776af2f2529009188fced91083203a48fd40#commitcomment-29024085 for prior discussion.

There have already been several changes made to the repo during the transition to Babel 7. This PR brings all tests back to green and allows us to move forward with the 0.56 branch cut.

We also bump our tests to use Xcode 9.4.0 and iOS 11.4, the latest stable versions of both.

Once the 0.56 branch makes it to stable, we can change `react-native-babel-preset@latest` on npm to point to `[email protected]` (or newer), and undo the change made to `init.js` we made as part of this diff.

Wait for Circle CI to run: https://circleci.com/workflow-run/e39a66d7-bf8a-4b31-a22f-eef30a2c53bc

[GENERAL] [BREAKING] [Babel] - Bump React Native Babel Preset version used by RN CLI to Babel v7 compliant release
Closes https://github.com/facebook/react-native/pull/19625

Reviewed By: TheSavior

Differential Revision: D8343861

Pulled By: hramos

fbshipit-source-id: 42644d5b0bfb40a8bc592ae3461c5008deef8232