Update Slim link (#725)

README.md

 * [Lumen](https://lumen.laravel.com) - A micro-framework by Laravel.
 * [Proton](https://github.com/alexbilbie/Proton) - A StackPHP compatible micro framework.
 * [Silex](http://silex.sensiolabs.org/) - A micro framework built around Symfony2 components.
-* [Slim](http://www.slimframework.com/) - Another simple micro framework.
+* [Slim](https://www.slimframework.com/) - Another simple micro framework.
 
 ## Micro Framework Extras
 *Extras related to micro frameworks and routers.*