[wp-trac] [WordPress Trac] #52666: Remove some lingering Travis CI references

[WordPress Trac]

#52666: Remove some lingering Travis CI references
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 Reporter:  johnbillion       |      Owner:  (none)
     Type:  task (blessed)    |     Status:  new
 Priority:  low               |  Milestone:  Awaiting Review
Component:  Build/Test Tools  |    Version:  trunk
 Severity:  normal            |   Keywords:  needs-patch
  Focuses:                    |
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 Previously: #50401

 There are a few lingering references to Travis CI that should be removed:

 * `travis:` tasks, `travisErrors`, and `travisWarnings` config in
 Gruntfile.js
 * A Build Status badge in README.md that comes from Travis via shields.io
 * Travis related skipping in the `skipOnAutomatedBranches()` test method

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Ticket URL: <https://core.trac.wordpress.org/ticket/52666>
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