fix: update travis build badge link

[thinkingserious]

Checklist

• I acknowledge that all my contributions will be made under the project's license

[codecov]

Codecov Report

Merging #151 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #151   +/-   ##
=======================================
  Coverage   98.27%   98.27%           
=======================================
  Files          27       27           
  Lines         810      810           
=======================================
  Hits          796      796           
  Misses         14       14

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