Remove remaining references to Travis CI

[jakob-keller]

Description of Change

Remove remaining references to Travis CI

Assumptions

Travis CI has not been used in four years and any references may safely be removed at this point.

Checklist for All Submissions

• I have added change info to CHANGES.rst
• If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
  • Detailed description of issue
  • Alternative methods considered (if any)
  • How issue is being resolved
  • How issue can be reproduced
• If this is providing a new feature (can be provided via link to issue with these details):
  • Detailed description of new feature
  • Why needed
  • Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• I have read and followed CONTRIBUTING.rst
• I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• I have ensured that the awscli/boto3 versions match the updated botocore version

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.00%. Comparing base (32808d2) to head (49185ad).
Report is 102 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1150      +/-   ##
==========================================
+ Coverage   86.88%   87.00%   +0.11%     
==========================================
  Files          63       63              
  Lines        5962     5963       +1     
==========================================
+ Hits         5180     5188       +8     
+ Misses        782      775       -7     

Flag	Coverage Δ
unittests	87.00% <100.00%> (+0.11%)	⬆️

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