Remove fork of Elixir-Slack

The upstream [BlakeWilliams/Elixir-Slack](https://github.com/BlakeWilliams/Elixir-Slack) has most, if not all, of the changes that were made in the [operable/Elixir-Slack](https://github.com/operable/Elixir-Slack). We should move over to using the upstream instead of using our own fork to keep up with improvements made to the library.