Use yarn

[cgewecke]

npm install

is hanging (on OSX & Node 10).

[codecov-io]

Codecov Report

Merging #332 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #332   +/-   ##
=======================================
  Coverage   92.07%   92.07%           
=======================================
  Files           6        6           
  Lines         366      366           
  Branches       79       79           
=======================================
  Hits          337      337           
  Misses         29       29

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