Replace gitter bade with slack

We don't use gitter anymore,

README.md

@@ -22,7 +22,7 @@
 [![devDependency Status](https://david-dm.org/sass/node-sass/dev-status.svg?theme=shields.io)](https://david-dm.org/sass/node-sass#info=devDependencies)
 [![Coverage Status](https://coveralls.io/repos/sass/node-sass/badge.svg?branch=master)](https://coveralls.io/r/sass/node-sass?branch=master)
 [![Inline docs](http://inch-ci.org/github/sass/node-sass.svg?branch=master)](http://inch-ci.org/github/sass/node-sass)
-[![Gitter chat](https://img.shields.io/badge/gitter-sass/node--sass-brightgreen.svg)](https://gitter.im/sass/node-sass)
+[![Join us in Slakc](https://libsass-slack.herokuapp.com/badge.svg)](https://libsass-slack.herokuapp.com/)
 
 Node-sass is a library that provides binding for Node.js to [LibSass], the C version of the popular stylesheet preprocessor, Sass.