have circle ci use xcode 9.4 (#19629)
have circle ci use xcode 9.4 (#19629)

Summary:
Circle CI just released Xcode 9.4 https://discuss.circleci.com/t/xcode-9-4-availability-on-circleci/22648/5

Make sure react native still works with Xcode 9.4.

None

[IOS][MINOR] Upgrade CI
Closes https://github.com/facebook/react-native/pull/19629

Differential Revision: D8343236

Pulled By: hramos

fbshipit-source-id: 2266aeafc9a1b1c77ad842f06c9a137bb05b135d

facebook/react-native,exponentjs/react-native,myntra/react-native,facebook/react-native,pandiaraj44/react-native,exponent/react-native,javache/react-native,javache/react-native,pandiaraj44/react-native,exponentjs/react-native,hoangpham95/react-native,exponentjs/react-native,exponentjs/react-native,arthuralee/react-native,javache/react-native,exponentjs/react-native,hoangpham95/react-native,arthuralee/react-native,hammerandchisel/react-native,myntra/react-native,javache/react-native,javache/react-native,hoangpham95/react-native,janicduplessis/react-native,myntra/react-native,pandiaraj44/react-native,exponent/react-native,javache/react-native,hammerandchisel/react-native,myntra/react-native,javache/react-native,javache/react-native,myntra/react-native,arthuralee/react-native,exponent/react-native,facebook/react-native,exponent/react-native,exponent/react-native,exponent/react-native,hoangpham95/react-native,myntra/react-native,myntra/react-native,hammerandchisel/react-native,exponent/react-native,pandiaraj44/react-native,hoangpham95/react-native,arthuralee/react-native,hoangpham95/react-native,facebook/react-native,arthuralee/react-native,hoangpham95/react-native,myntra/react-native,hammerandchisel/react-native,myntra/react-native,exponent/react-native,pandiaraj44/react-native,pandiaraj44/react-native,janicduplessis/react-native,pandiaraj44/react-native,facebook/react-native,facebook/react-native,janicduplessis/react-native,hammerandchisel/react-native,javache/react-native,hoangpham95/react-native,exponentjs/react-native,hammerandchisel/react-native,facebook/react-native,janicduplessis/react-native,janicduplessis/react-native,facebook/react-native,exponentjs/react-native,exponentjs/react-native,hammerandchisel/react-native,janicduplessis/react-native,pandiaraj44/react-native,facebook/react-native,janicduplessis/react-native,janicduplessis/react-native,hammerandchisel/react-native