Force requests to always use https

A recent change to http-browserify defaults the scheme (http or https) for requests to the current page's scheme - https://github.com/substack/http-browserify/commit/e4a5316f34de17334df90bc5f20dc654c5fcc636

This would break sites that were using mixpanel-node on a https website, because Mixpanel does not support https.