update-check doesn't use proxy from environment #14

Not sure if this is one for here or https://github.com/zeit/serve but it looks like update-check doesn't pick up the proxy from the $HTTP_PROXY/$HTTPS_PROXY env vars, or anywhere else as far as I can tell (possibly because http/https don't do it automatically?)
As a result serve takes a very long time before the update check times out when behind a proxy, which I'm sure it didn't used to? Much as in an ideal world there wouldn't be any proxies, the reality of corporate networks is far from ideal!

I've tried this on Ubuntu 16.04.1 (node v8.12.0) and macOS 10.13.6 (node v9.11.2)

serve is a brilliant little tool by the way!