Support running on remote machine

Running on a remote machine (e.g. some server) mostly works, but the web UI
doesn't refresh automatically because socket.io is hard-coded to
'http://localhost'.

There's no need to hard-code this - if you just use socket.io's default then
it works for localhost and/or a remote server too, depending on which URL
you typed in the browser to load the MailDev UI.