portalHelper: Address intermittent TLS failures

Users are currently experiencing intermittent (and hard to debug) TLS
errors when loading the portal authentication page. This could be
caused by using http://www.gnome.org/ for our connectivity checks
because it redirects to HTTPS.

We can completely remove TLS from the equation by not using HTTPS.
http://nmcheck.gnome.org/ is a good choice because it doesn't redirect
to HTTPS and was created for NetworkManager's connectivity check.

https://bugzilla.gnome.org/show_bug.cgi?id=769940