Don't do the port threshold check if running under root

[samerbuna]

For the rare case where a risk-taker wants to run the machine under root (and thus have access to ports under 1024), allow them.

It's fairly important in our setup that we forward ports 80 and 443 to vagrant, running the machine under root is an acceptable compromise.

[mitchellh]

I merged in your pull request and also made a slight modification:

Its not safe to simply check for "root" in the ENV["USER"], since this won't work on Windows and won't work in some shells since the "USER" env var might not be sent. In the end, I concluded there is no safe way to check this (that I know of), so instead of prohibiting it, I simply issue a warning.

This would be fine :)