Upgrade mocha

[wong2]

upgrade mocha to eliminate these warnings:

npm WARN deprecated to-iso-string@0.0.2: to-iso-string has been deprecated, use @segment/to-iso-string instead.
npm WARN deprecated jade@0.26.3: Jade has been renamed to pug, please install the latest version of pug instead of jade
npm WARN deprecated minimatch@0.3.0: Please update to minimatch 3.0.2 or higher to avoid a RegExp DoS issue

[codecov]

Codecov Report

Merging #900 into v2.x will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff         @@
##           v2.x   #900   +/-   ##
===================================
  Coverage   100%   100%           
===================================
  Files         4      4           
  Lines       421    421           
  Branches    100    100           
===================================
  Hits        421    421

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