docs: fixed badge

Author: niftylettuce

README.md

@@ -103,9 +103,9 @@ This would output the following for the previous example:
 
 [standard-url]: https://github.com/crocodilejs/eslint-config-crocodile
 
-[slack-image]: http://slack.crocodilejs.com/badge.svg
+[slack-image]: https://slack.crocodilejs.com/badge.svg
 
-[slack-url]: http://slack.crocodilejs.com
+[slack-url]: https://slack.crocodilejs.com
 
 [mongoose]: https://github.com/Automattic/mongoose