updates to the latest v3.x @slack/client

[aoberoi]

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #529 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #529   +/-   ##
=======================================
  Coverage   85.55%   85.55%           
=======================================
  Files           6        6           
  Lines         353      353           
  Branches       79       79           
=======================================
  Hits          302      302           
  Misses         29       29           
  Partials       22       22

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 76a03bc...498ffe4. Read the comment docs.

[aoberoi]

Blocking on internal conversation about how to handle incompatibility with node v0.12.