Update URL for gitignore.io

The old, non-https URL results in a 301 document for any `gi` command:

```html
<html>
<head><title>301 Moved Permanently</title></head>
<body bgcolor="white">
<center><h1>301 Moved Permanently</h1></center>
<hr><center>cloudflare-nginx</center>
</body>
</html>
```