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

.travis.yml

@@ -3,7 +3,7 @@ language: objective-c
 osx_image: xcode8.3
 
 install:
-  - nvm install 7
+  - nvm install 8
   - rm -Rf "${TMPDIR}/jest_preprocess_cache"
   - brew install yarn --ignore-dependencies
   - brew install watchman