Update to node 10 in .travis.yml

[msimerson]

closes #40

[codecov]

Codecov Report

Merging #41 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #41   +/-   ##
=======================================
  Coverage   81.54%   81.54%           
=======================================
  Files           8        8           
  Lines         428      428           
  Branches      124      124           
=======================================
  Hits          349      349           
  Misses         79       79

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