(fix) add bundlesize to travis fixes #46

[potench]

• Adds bundlesize npm script to travis CI
• Updates node version in travis CI to latest stable node. https://docs.travis-ci.com/user/languages/javascript-with-nodejs/#Specifying-Node.js-versions

[codecov]

Codecov Report

Merging #48 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #48   +/-   ##
=======================================
  Coverage   93.04%   93.04%           
=======================================
  Files           8        8           
  Lines         575      575           
=======================================
  Hits          535      535           
  Misses         40       40

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