Changed config.vm.box to use Vagrant Cloud box

Error:
-------

````
==> db: Box 'ubuntu-precise64' could not be found. Attempting to find and install...
    db: Box Provider: virtualbox
    db: Box Version: >= 0
The box '["box/precise64.box"]' could not be found or
could not be accessed in the remote catalog. If this is a private
box on Vagrant Cloud, please verify you're logged in via
`vagrant login`. Also, please double-check the name. The expanded
URL and error message are shown below:

URL: ["https://vagrantcloud.com/box/precise64.box"]
Error: The requested URL returned error: 404 Not Found
````

Fix:
----

Upgraded `config.vm.box` to use a new(er) [Vagrant Cloud shorthand name](https://docs.vagrantup.com/v2/cli/box.html) instead.