Fixes for rtm.start deprecation

[iclaxton]

I have been unable to test all the changes in this update but I attempted maintain the interface. Clearly this lib has a shelf life as slack want people to move to their maintained libs written in Java, JS & Python.

I timeboxed this update so some of the code is not tested and have not run any unit tests as I do not have access to a non-business slack environment.

Hopefully if this passes it might help keep this lib alive for a bit longer.