Remove .travis.yml

[jtpio]

Fixes #556

[jtpio]

cc @SylvainCorlay @maartenbreddels @martinRenou

I remember there were some concerns to fully switch to GitHub Actions when we first started to use them for the repo. However it has now been a couple of months and although random failures might still happen, they seem to be happening less often than on Travis.

Dropping Travis would also help maintain only one CI workflow.

[jtpio]

For the record, we discussed this over video and there was no objection to dropping Travis.

So let's do it.