fix Code Climate badge on README.md (#5671)

and change badge's URI to https

README.md

 ![Mastodon](https://i.imgur.com/NhZc40l.png)
 ========
 
-[![Build Status](http://img.shields.io/travis/tootsuite/mastodon.svg)][travis]
-[![Code Climate](https://img.shields.io/codeclimate/github/tootsuite/mastodon.svg)][code_climate]
+[![Build Status](https://img.shields.io/travis/tootsuite/mastodon.svg)][travis]
+[![Code Climate](https://img.shields.io/codeclimate/maintainability/tootsuite/mastodon.svg)][code_climate]
 
 [travis]: https://travis-ci.org/tootsuite/mastodon
 [code_climate]: https://codeclimate.com/github/tootsuite/mastodon