Switch to fork of supertest

[bkeepers]

ladjs/supertest#446 should help reveal the underlying source of errors with supertest. Here's what it looks like now:

Before:

Cannot read property 'status' of undefined

After:

NetConnectNotAllowedError: Nock: Not allow net connect for "127.0.0.1:56508/slack/command"

[codecov]

Codecov Report

Merging #118 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #118   +/-   ##
=======================================
  Coverage   72.78%   72.78%           
=======================================
  Files          28       28           
  Lines         474      474           
  Branches       65       65           
=======================================
  Hits          345      345           
  Misses        114      114           
  Partials       15       15

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