Use https://www.virtualbox.org for all VirtualBox links

From an internal ticket:

> Just noticed that the Link to Virtualbox points to the not-www-Subdomain. And on Domain i got a https-error. If you change the link to the www-Subdomain, there’s no error.
Tune HTML minification
Merge pull request #4686 from mitchellh/sethvargo/vb_ssl

website: Use https://www.virtualbox.org for all VirtualBox links