Remove leftover Travis CI references

[tvainika]

Changes

Remove leftover Travis CI references, which currently lead to error pages.

Checklist

• [X] I think the code is well written
• [X] Unit tests for the changes exist
• [X] Documentation reflects the changes
• [ ] Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[codecov[bot]]

Codecov Report

Merging #856 (e195be6) into master (6360747) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #856   +/-   ##
=======================================
  Coverage   97.55%   97.55%           
=======================================
  Files          30       30           
  Lines        5450     5450           
=======================================
  Hits         5317     5317           
  Misses        133      133           

Flag	Coverage Δ
cext	88.40% <ø> (ø)
integration	97.52% <ø> (ø)
purepy	97.06% <ø> (ø)
unit	38.09% <ø> (ø)

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