Update Vagrant file

I don't have a `base` box locally; should I? 
As is, the demo app won't `vagrant up`:

```console  
$ vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Box 'base' could not be found. Attempting to find and install...
    default: Box Provider: virtualbox
    default: Box Version: >= 0
==> default: Adding box 'base' (v0) for provider: virtualbox
    default: Downloading: base
An error occurred while downloading the remote file. The error
message, if any, is reproduced below. Please fix this error and try
again.

Couldn't open file /Users/clint/Atlas/atlas-example/app/base
```

Updated to `hashicorp/precise64`, seemed a reasonable default.