Retire Travis, in favor of GitHub Actions

[jminor]

Link the Issue(s) this Pull Request is related to.

Fixes #788

Summarize your change.

Travis has served us well, but now that we have switched to GitHub Actions, we don't need both.
This PR removes the Travis config file, retiring OTIO's use of Travis.

[codecov-io]

Codecov Report

Merging #817 into master will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #817      +/-   ##
==========================================
+ Coverage   84.02%   84.18%   +0.15%     
==========================================
  Files          74       74              
  Lines        3124     3060      -64     
==========================================
- Hits         2625     2576      -49     
+ Misses        499      484      -15     

Flag	Coverage Δ
#py27	?
#py37	?
#py38	?
#unittests	84.18% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ntimelineio/opentimelineio-bindings/otio_utils.cpp	86.90% <0.00%> (-0.46%)	⬇️
src/opentimelineio/typeRegistry.cpp	84.84% <0.00%> (-0.45%)	⬇️
...entimelineio-bindings/otio_serializableObjects.cpp	95.11% <0.00%> (-0.31%)	⬇️
src/opentimelineio/item.cpp	90.47% <0.00%> (-0.15%)	⬇️
...lineio/opentime-bindings/opentime_rationalTime.cpp	97.36% <0.00%> (-0.07%)	⬇️
src/opentimelineio/serialization.cpp	83.60% <0.00%> (-0.06%)	⬇️
src/opentimelineio/timeline.h	100.00% <0.00%> (ø)
src/opentimelineio/typeRegistry.h	100.00% <0.00%> (ø)
...imelineio/opentime-bindings/opentime_timeRange.cpp	100.00% <0.00%> (ø)
...melineio/opentimelineio-bindings/otio_bindings.cpp	100.00% <0.00%> (ø)
... and 11 more

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[ssteinbach]

Thanks!