Use Node 8 instead of 7 in Travis CI
Use Node 8 instead of 7 in Travis CI

Summary:
Node 8 is stable and newer and also gives us test coverage for npm 5, which is probably better to use for the E2E tests.
Closes https://github.com/facebook/react-native/pull/15904

Differential Revision: D5814784

Pulled By: hramos

fbshipit-source-id: 5b57293e804e1626fd20c200ed54c7210a6ad915

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