Old packs are not deleted at start of run

[macobo]

Currently, when you run this gem, it doesn't clean up old assets from public/packs on start of run, and all of its contents get rsync-ed. This can mean accidentally syncing over a GB of data if the folder hasn't been cleaned in a while.

The problem comes from this: at start of run, rake assets:clean is called, which doesn't clear packs dir, but the rm -rf later on does.