Fixed permissions for apache

I was using this as an example in a new Panamax template and my app was having trouble writing to the folder and thought this repo could benefit from it too. When I tested this I noticed WordPress could not do any updates to itself or plugins which usually means permission issues and I think it is because Apache runs with a specific user `www-data` and group `www-data` and they do not have access as the `/app` directory is owned by `root`.

What do you think @patocox?