README: point out you need new vagrant
fix(serial): Disable serial logging by default

The serial console logging causes VirtualBox to consume an immense
amount of CPU, making coreos-vagrant nearly unusable. Disabling it
by default treats the problem until VirtualBox can fix the problem
upstream.
Merge pull request #85 from bcwaldon/disable-serial

Disable serial logging by default
feat(config): Use config.rb for configuration