Migrate from Travis CI
Following the discussion at Gitter (https://gitter.im/kaitai_struct/Lobby?at=5f9fe0f72a60f731f72da5b7):

> FYI, Travis CI now no longer provides unlimited CI for open-source projects, starting yesterday. Which they announce in a blog post today? https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing

> it might be easier to just migrate everything to GitHub Actions/CircleCI/whatever instead

Cc @dgelessus, @GreyCat 

I'll put a checklist here tracking the progress _(based on https://github.com/search?q=org%3Akaitai-io+path%3A**%2F.travis.yml&type=code)_:

- [x] [**ci_targets** / .travis.yml](https://github.com/kaitai-io/ci_targets/blob/4e2830c/.travis.yml)
- [x] [**kaitai_struct** / .travis.yml](https://github.com/kaitai-io/kaitai_struct/blob/4473547/.travis.yml)
- [x] [**kaitai_struct_doc** / .travis.yml](https://github.com/kaitai-io/kaitai_struct_doc/blob/f4bf35e/.travis.yml)
- [x] [**kaitai_struct_webide** / .travis.yml](https://github.com/kaitai-io/kaitai_struct_webide/blob/5b1ec3f/.travis.yml)
- [x] [**awesome-kaitai** / .travis.yml](https://github.com/kaitai-io/awesome-kaitai/blob/13d1834/.travis.yml)
- [x] [**kaitai_ci_ui** / .travis.yml](https://github.com/kaitai-io/kaitai_ci_ui/blob/f89dce4/.travis.yml)