Remove proxy settings from defaults

The `mikeal/request` package already handles proxy support, including environment variables `http_proxy`, `https_proxy`, `no_proxy`, and variants. `npm` should let `request` take care of proxying based on environment variables and only set proxy values if `.npmrc` has a proxy specified. By passing a proxy to request, request will always use that proxy setting and will not honor `no_proxy` settings. This will enable the use of a private npm repository that is within a corporate firewall.
 
See https://github.com/npm/npm/issues/6524 for more details.