Simplify coverage upload in .travis.yml

[blegat]

See https://docs.travis-ci.com/user/languages/julia/#coverage

[codecov-io]

Codecov Report

Merging #795 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #795   +/-   ##
======================================
  Coverage    94.1%   94.1%           
======================================
  Files          59      59           
  Lines        6671    6671           
======================================
  Hits         6278    6278           
  Misses        393     393

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