Upgrade to slackclient 2.x

[jvperrin]

slackclient 2.x introduces a number of breaking changes (like splitting the existing client into a RTMClient and a WebClient) which we need to implement when upgrading to that version in this project. There's an existing guide on migrating, but it'll need some careful work and testing to make sure everything works once the upgrade happens. Adding tests (as in #50) would also help to make this transition much smoother.