Rename box to "adbox"

This is for convenience so that the name that Vagrant uses matches what we'd expect, and avoids collisions with other boxes named "default."

However, it's somewhat of a breaking change; the old "default" box will be orphaned and can't be torn down after this change is committed, without one of these workarounds:

##Workaround 1
1. Don't git pull yet, or roll back to the commit before this
2. $ `cd /path/to/boxes/adbox ; vagrant destroy`
3. $ `git pull`
4. $ `vagrant up`

##Workaround 2

1. Edit Vagrantfile and in line 5 (`config.vm.define :adbox do |ae_config|`), change `adbox` to `default`.
2. $ `cd /path/to/boxes/adbox ; vagrant destroy`
3. Revert your change to Vagrantfile
4. $ `vagrant up`