Rename iOS module name of sample app from gutenberg to GutenbergDemo.

[SergioEstevao]

With the update to React 0.61.5 the iOS demo project is now built using CocoaPods.

This creates a scenario where sometimes Xcode build system gets confused between the pod library Gutenberg (the Gutenberg Bridge podspec) and the binary of the demo project itself: gutenberg.

You will see Xcode sometimes complaining that we are importing Gutenberg instead of gutenberg, and after getting on this state you will probably need to clean all build folders and restart the project.

So the solution I propose here is to rename the module name of the gutenberg.xcodeproj from gutenberg to GutenbergDemo to avoid these conflicts going forward.

To test:

• Check if projects build
• Check that tests build and pass

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[koke]

Makes sense, and seems to work fine after cleaning the build folder 👌

[hypest]

👋 @SergioEstevao , as a follow up, do you think that we could/should also rename the local files that are still named as "gutenberg.xxx"? For example, all the files/folders in https://github.com/wordpress-mobile/gutenberg-mobile/tree/develop/ios are still named/prefixed gutenberg.