Add a task to update apt-get on boot in ubuntu.

Bento boxes ship with package indexes that become stale. When the indexes are stale, package installation fails until the index is manually refreshed. This breaks the various Chef provisioners. I documented another approach to addressing this problem here:

https://gist.github.com/whilp/5430496

It seems preferable to deal with this in bento, though, to avoid copypasta/drift in Vagrantfiles.