Add specs for Rex::Oui


Verification
----------

- [ ] Be sure which travis is green
- [ ] Run the specs locally and ensure all the tests pass

```
$ rspec spec/lib/rex/mac_oui_spec.rb



Rex::Oui ..........

Finished in 0.01826 seconds
10 examples, 0 failures
```
http://blog.davidchelimsky.net/blog/2012/05/13/spec-smell-explicit-use-of-subject/