firebase-ios-sdk

Build Firestore with Xcode 11 (#4005)

* Drop dependency on OCMock

* Move Firestore onto Xcode 11 in Travis

* Special case simulators based on Xcode version, not product

This allows Firestore's build with Xcode 8.3.3 to continue to work.

* Make the Firestore Podfile install only one platform at a time

Use the PLATFORM environment variable to limit which targets are
modified. This makes the selected platform work but leaves the other
platforms in a broken state in the Xcode UI.

* Fix code signing errors on macOS builds