Remove references to Spotify FOSS Slack

The FOSS Slack is being shut down. We will continue use GitHub issues, discussions, etc. for receiving feedback and bug reports.

README.md

@@ -5,7 +5,6 @@
 [![License](https://img.shields.io/github/license/spotify/SPTDataLoader.svg)](LICENSE)
 [![CocoaPods](https://img.shields.io/cocoapods/v/SPTDataLoader.svg)](https://cocoapods.org/pods/SPTDataLoader)
 [![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)
-[![Spotify FOSS Slack](https://slackin.spotify.com/badge.svg)](https://slackin.spotify.com)
 [![Readme Score](http://readme-score-api.herokuapp.com/score.svg?url=https://github.com/spotify/sptdataloader)](http://clayallsopp.github.io/readme-score?url=https://github.com/spotify/sptdataloader)
 
 Authentication and back-off logic is a pain, let's do it once and forget about it! This is a library that allows you to centralise this logic and forget about the ugly parts of making HTTP requests.