chore: Remove Slack badge

Author: nschonni

README.md

@@ -40,7 +40,6 @@ Node <8 | <5.0                        | <57
 [![Windows x86](https://ci.appveyor.com/api/projects/status/22mjbk59kvd55m9y/branch/master?svg=true)](https://ci.appveyor.com/project/sass/node-sass/branch/master)
 [![npm version](https://badge.fury.io/js/node-sass.svg)](http://badge.fury.io/js/node-sass)
 [![Coverage Status](https://coveralls.io/repos/sass/node-sass/badge.svg?branch=master)](https://coveralls.io/r/sass/node-sass?branch=master)
-[![Join us in Slack](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.