Remove references to Travis CI

[HertzDevil]

Following crystal-lang/crystal#10690 and https://forum.crystal-lang.org/t/migrate-ci-in-crystal-lang-projects/3363, we no longer officially support Travis CI anymore. This PR removes any references to it in the guides.

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[straight-shoota]

Just to be clear, there are two independent aspects and neither of them is really about Crystal supportin travis:

• The default travis.yml in crystal init. That has been removed. Thus, references should be removed from the book as well.
• The guide about using Crystal with travis. This requires travis to support Crystal which really isn't the case anymore, at least not with language: crystal (see Update repository for crystal to OBS travis-ci/travis-build#1987). You could probably still use Crystal on travis with docker. But the guide isn't about that. So it's probably best to remove it.