ci: drop Travis

[Xyene]

Travis has not been run in the last 4 months.

[codecov-commenter]

Codecov Report

Merging #955 (da45af3) into master (08e69d7) will decrease coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #955      +/-   ##
==========================================
- Coverage   86.54%   86.29%   -0.26%     
==========================================
  Files         138      138              
  Lines        4735     4735              
==========================================
- Hits         4098     4086      -12     
- Misses        637      649      +12     

Impacted Files	Coverage Δ
dmoj/cptbox/tracer.py	83.27% <0.00%> (-1.82%)	⬇️
dmoj/executors/base_executor.py	83.67% <0.00%> (-1.71%)	⬇️
dmoj/result.py	89.33% <0.00%> (-1.34%)	⬇️
dmoj/executors/gcc_executor.py	91.83% <0.00%> (-1.03%)	⬇️

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