Update code to use arrow functions, let/const vars, etc from ES 2017

[nkramaric]

Hello! I just started using this botkit-starter and it has been extremely helpful!

One thing I did when I started working with it in my repo was to upgrade to the latest ES syntax with arrow functions and let/const. Maybe this is something that others would like updated as well? I can go ahead and send a PR if this is something that you would find useful.

[peterswimm]

Hey - there are some careful considerations we would need to make to approve a PR request of this, but by all means feel free to submit one.