Add .circleci/config.yml

[issmirnov]

Set up config for https://app.circleci.com/pipelines/github/issmirnov builds.

Travis is limiting me to low credits that run out quickly. Migrating to alternatives. Solves #36

[codecov]

Codecov Report

Merging #37 (f9f18d9) into master (a3ced18) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #37   +/-   ##
=======================================
  Coverage   62.84%   62.84%           
=======================================
  Files           4        4           
  Lines         183      183           
=======================================
  Hits          115      115           
  Misses         60       60           
  Partials        8        8           

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