replace gitter with slack

[bassam]

We should replace the gitter icon and link with slack:

<script async defer src="https://slack.yourdomain.com/slackin.js"></script>
<!-- append "?" to the URL for the large version -->

See https://github.com/rauchg/slackin#badges

our domain is https://rook-slackin.herokuapp.com so it might be:

<script async defer src="https://rook-slackin.herokuapp.com/slackin.js"></script>
<!-- append "?" to the URL for the large version -->

[ilovett]

Fixed via #39