Switch back to GitHub Actions before my Travis-CI subscription renews

[defuse]

I had to pay for Travis-CI to fix the build tests so let's see if we can switch everything to GitHub Actions.

[defuse]

It's now kinda hard to find the old github actions config because I deleted it so here's the link: https://github.com/defuse/php-encryption/blob/90060874f868d0711890e67456b4f26930fc478f/.github/workflows/ci.yml