Update project to XCode 9.3. Rename flatMap to compactMap

[dcoufal]

Description

The PR updates the project to Xcode 9.3.

Change Notes

• Added recommended project changes from Apple.
• Renames flatMap to compactMap

Pre-submission Checklist

• [x] I ran the unit tests locally before checking in.
• [x] I made sure there were no compiler warnings before checking in.
• [x] I have written useful documentation for all public code.
• [ ] I have written unit tests for this new feature.

[dcoufal]

Waiting for Travis CI to update to a Xcode 9.3 image...