Vagrant 1.3 compatibility

Thank you to publish this sample.

When I ran vagrant up and get the following error.
(I tried Vagrant 1.3.4)

```
There are errors in the configuration of this machine. Please fix
the following errors and try again:

vm:
* Shared folder mount options specified by 'mount_options' must
be an array of options.
```

can not use `:extra` from 1.3.0.
https://github.com/mitchellh/vagrant/blob/master/CHANGELOG.md#130-september-5-2013

So, I tried to fix. Thanks in advance.