Maintaining Environment Variables with sudo Raw

Especially useful if you need to use proxies. E.g., this will fail since
the `sudo` user doesn't know about the `https_proxy` variable.

    sudo curl -L https://get.rvm.io | bash -s stable --ruby

The solution is to `visudo`, find this

    Defaults env_reset 

and add this for each variable you want available.

    Defaults env_keep += "variable1 variable2"

In the case of proxies,

    Defaults env_keep += "http_proxy https_proxy ftp_proxy"