Validate vhosts names more strictly

There was a bug in the setup page where valid domains that start with a number
would be captured by the validation check designed to detect IP
addresses. This check detects IPs by comparing the host name to a regex,
instead of simply detecting whether the first character is a number.

contrib/dokku-installer.rb

@@ -104,7 +104,7 @@ __END__
 				});
 		}
 		function update() {
-			if ($("#vhost").is(":checked") && !isNaN($("#hostname").val()[0])) {
+			if ($("#vhost").is(":checked") && $("#hostname").val().match(/(\d{1,3}\.){3}\d{1,3}/)) {
 				alert("In order to use virtualhost naming, the hostname must not be an IP but a valid domain name.")
 				$("#vhost").prop('checked', false);
 			}