Fix TLS events

Event monitoring is not working under TLS because it isn't using Client.HTTPClient. This patch fixes it.

I would have added a test, but `httptest.NewTLSServer` doesn't support TLS mutual authentication and my skills are not up to writing a full replacement.
This method of choosing a scheme should probably be overridable through the environment, and the port should probably be determined by `net.SplitHostPort(u.Host)` - for example: https://github.com/rafecolton/docker-builder/blob/master/dclient/dclient.go#L172-L186