command: Use full URL for the download page when prompting for upgrade

This makes it more likely that a terminal emulator will detect the URL as a clickable link.

command/version.go

@@ -108,7 +108,7 @@ func (c *VersionCommand) Run(args []string) int {
 		if info.Outdated {
 			c.Ui.Output(fmt.Sprintf(
 				"\nYour version of Terraform is out of date! The latest version\n"+
-					"is %s. You can update by downloading from www.terraform.io/downloads.html",
+					"is %s. You can update by downloading from https://www.terraform.io/downloads.html",
 				info.Latest))
 		}
 	}