Update badges and formatting

[atouchet]

• Replace Travis CI badge with GitHub Actions and remove references

• Fix crates.io badge

• Update some formatting and use HTTPS for links

• I agree to follow the project's code of conduct.

• I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

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[lnicola]

Thanks, looks good. Do you think a docs.rs badge would be useful? I like them, but a lot of projects do without. Something like:

[![API docs]][docs.rs]

[API docs]: https://docs.rs/tilejson/badge.svg
[docs.rs]: https://docs.rs/tilejson/

[atouchet]

Either option seems fine to me. Do you want a docs.rs badge added?

[lnicola]

I only have commit rights here incidentally.

r? @stepankuzmin

[stepankuzmin]

Yep, that would be great, thanks!

[atouchet]

Okay I've added a docs badge.