Install prerequisites for generating proto sources

Author: wilhuff

scripts/install_prereqs.sh

@@ -56,6 +56,9 @@ case "$PROJECT-$PLATFORM-$METHOD" in
   Firestore-*-cmake)
     brew outdated cmake || brew upgrade cmake
     brew outdated go || brew upgrade go # Somehow the build for Abseil requires this.
+
+    # Install python packages required to generate proto sources
+    pip install six
     ;;
 
   *)