Use React Native Packager xcode script for build phase

[aaronschachter]

Refs #752 - #765 seemed to get a build phase compiling main.jsbundle working for me, but not others (#777). It seems like the code I left out of the build phase (which I was copying from node_modules/react-native/packager/react-native-xcode.sh) is important -- I suspect it's required the relevant JS properly on the simulator.

The build phase code is from new changes made here to local-cli/generator-ios/templates/xcodeproj/project.pbxproj: facebook/react-native#5518

This PR only works by manually updating your node_modules/react-native/packager/react-native-xcode.sh with the latest changes per that PR (from today): https://github.com/facebook/react-native/blob/d3e44143f6643662432960d156e93c3e1a49781d/packager/react-native-xcode.sh. Pasting the code into your local should get the build script to run. Hacking the packager script locally should no longer be needed upon the next 0.19.0 release (though we'll need to update our package.json dependencies as per changes in this PR). Will keep #777 open until then.

Note: moved index.io.js to the root directory to make this work out of box better -- the react-native-xcode.sh file always seems to look at index.ios.js (having trouble specify the ReactComponent folder without modifying the .sh script locally)

cc @jonuy @lkpttn

[aaronschachter]

0.14.0 introduced the xcode packager script: http://facebook.github.io/react-native/docs/upgrading.html#from-0-13-to-0-14 - we also don't need the main.jsbundle at all per those docs. Removing, and will update wiki once merged

[aaronschachter]

Walked through manually updating the react-native-xcode.sh with @lkpttn and got builds working on his machine. 🎆

Will keep #777 open until React Native 0.19.0 is released and we don't need to manually alter the script locally.