Fix build on Travis CI and add Python 3.7 for the build

[hayatoikoma]

Fixed build on Travis CI.

I'm not sure if Python 3.5 should be dropped, but I added Python 3.7 for the build.

[codecov-io]

Codecov Report

Merging #14 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #14   +/-   ##
=======================================
  Coverage   87.58%   87.58%           
=======================================
  Files           4        4           
  Lines         282      282           
=======================================
  Hits          247      247           
  Misses         35       35           

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