update build badge to use Github Actions

[noviluni]

We stopped using Travis in favor of Github Actions, so I changed the badge from Travis to Github Actions.

Result preview: https://github.com/scrapinghub/dateparser/blob/779e1e8e1632acab75c5d73cd24e854cd79b69cf/README.rst

[codecov]

Codecov Report

Merging #884 (3456b83) into master (be05e0e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #884   +/-   ##
=======================================
  Coverage   98.20%   98.20%           
=======================================
  Files         229      229           
  Lines        2504     2504           
=======================================
  Hits         2459     2459           
  Misses         45       45           

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