Advertise our slack channel

[solidnerd]

I created a link for our slack channel on elixir-lang. I hope you like it.

[solidnerd]

Travis seems to be broken but it has something todo with The command "bundle exec rake site:deploy --quiet" exited with 1.

[jnbt]

1. As we switch over to Slack I think we should drop Gitter (two chats are way too much).
2. I think we need to find a solution to link to the "Invite page"

/cc @JanOwiesniak @splattael

[jnbt]

On a side note: Travis is failing as for external PRs the encrypted environment variables are not present (which makes totally sense due for security). I added @solidnerd as a member so he should be able to create internal PRs