Allow disabling of proxies with proxy: false

With the current setup in Excon, it's more complicated to opt-out of proxies when they are set through `ENV['HTTPS_PROXY']`, since you have to explicitly whitelist the host through another `ENV` variable. It's a lot easier to do it when we create our Excon config by setting `proxy: false`.

Alternatively, if you want to be sure there isn't a regression, I could change it to something like `disable_proxies: true`.