Now that I've found the save game location on the Mac, I know exactly what happened. Yosemite deletes all cache files and logs during the install. Since ./Library/Caches/unity.Phoenix Online Studios.GK1/001_St. George's Books.sav is in ./Library/Caches, it gets blown away during the upgrade.
Save games should be stored in a more "usual" location on the Mac, like ./Library/Application Support or ./Documents.
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *path = [paths objectAtIndex:0];