Replace references to Gitter with Slack

The [main page](https://github.com/nodegit/nodegit.github.com/blame/832700d77d76f11f3591023f837d072df6d7e46d/index.md#L131-L136) (and possibly other pages) have references to Gitter. They should be replaced with links to the #nodegit channel running on libgit2's Slack instance.