Use https to get the latest docker version

To avoid unexpected results since docker was using http.
For instance, my broadband doesn't return not found when it's down but
a html page saying that the internet is down. Docker was showing that
html instead of ignoring it.

Fix #3570

Docker-DCO-1.1-Signed-off-by: Fabio Falci <fabiofalci@gmail.com> (github: fabiofalci)

utils/utils.go

@@ -827,7 +827,7 @@ func ParseHost(defaultHost string, defaultPort int, defaultUnix, addr string) (s
 }
 
 func GetReleaseVersion() string {
-	resp, err := http.Get("http://get.docker.io/latest")
+	resp, err := http.Get("https://get.docker.io/latest")
 	if err != nil {
 		return ""
 	}