docs: remove Travis CI build status badge from README

[tangcent]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.077%. Comparing base (0f706c1) to head (394d026).

Additional details and impacted files

@@               Coverage Diff               @@
##              master     #1136       +/-   ##
===============================================
+ Coverage     62.073%   62.077%   +0.004%     
- Complexity      2289      2291        +2     
===============================================
  Files            250       250               
  Lines          12126     12130        +4     
  Branches        2996      2996               
===============================================
+ Hits            7527      7530        +3     
- Misses          2994      2995        +1     
  Partials        1605      1605               

Flag	Coverage Δ
unittests	62.077% <ø> (+0.004%)	⬆️

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see 4 files with indirect coverage changes

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