Restore cmake build inadvertently deleted in #2482 (#2948)

Author: paulb777

.travis.yml

@@ -161,6 +161,13 @@ jobs:
         - travis_retry ./scripts/build.sh $PROJECT $PLATFORM $METHOD
 
     # Alternative platforms
+    - stage: test
+      env:
+        - PROJECT=Firestore PLATFORM=macOS METHOD=cmake
+      before_install:
+        - ./scripts/if_changed.sh ./scripts/install_prereqs.sh
+      script:
+        - travis_retry ./scripts/if_changed.sh ./scripts/build.sh $PROJECT $PLATFORM $METHOD
 
     # Test Firestore on Xcode 8 to use old llvm to ensure C++ portability.
     - stage: test