Replace gitter with slack

We are using slackin (https://github.com/rauchg/slackin) to automatically accept people's invitation.

README.md

@@ -3,7 +3,8 @@
 
 > The <a href="http://fishshell.com">Fishshell</a> Framework
 
-[![MIT License](https://img.shields.io/badge/license-MIT-007EC7.svg?style=flat-square)](/LICENSE.md) [![Fish Shell Version](https://img.shields.io/badge/fish-v2.2.0-007EC7.svg?style=flat-square)](http://fishshell.com) [![Travis Build Status](http://img.shields.io/travis/oh-my-fish/oh-my-fish.svg?style=flat-square)](https://travis-ci.org/oh-my-fish/oh-my-fish) [![Travis Build Status](https://img.shields.io/badge/gitter-join_chat-brightgreen.svg?style=flat-square)](https://gitter.im/oh-my-fish/oh-my-fish?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![MIT License](https://img.shields.io/badge/license-MIT-007EC7.svg?style=flat-square)](/LICENSE.md) [![Fish Shell Version](https://img.shields.io/badge/fish-v2.2.0-007EC7.svg?style=flat-square)](http://fishshell.com) [![Travis Build Status](http://img.shields.io/travis/oh-my-fish/oh-my-fish.svg?style=flat-square)](https://travis-ci.org/oh-my-fish/oh-my-fish) [![Slack Status](https://oh-my-fish-slack.herokuapp.com/badge.svg)](https://oh-my-fish-slack.herokuapp.com)
+
 
 Oh My Fish provides core infrastructure to allow you to install packages which extend or modify the look of your shell. It's  fast, extensible and easy to use.
 
@@ -38,7 +39,7 @@ We just moved Oh My Fish repository back to our own organization. Since then, yo
 8. Convert all `Theme "name"` lines to `theme name`
 9. `omf install`
 
-If you find any trouble in the upgrade process join us in [Gitter](https://gitter.im/oh-my-fish/oh-my-fish) and we will be happy to help.
+If you find any trouble in the upgrade process join us in [Slack](https://oh-my-fish-slack.herokuapp.com) and we will be happy to help.
 
 # Getting Started