Switch to GitHub Actions.

[demiankatz]

TODO

• [x] Make sure build is actually working.

[demiankatz]

This PR switches the project from Travis to GitHub actions for continuous integration. I believe it is functionally equivalent to the old configuration in most regards.

Some notes:

• I had to update some Composer dependencies to make the build pass, but I only made the bare minimum changes to dev-only dependencies, so I think this should be harmless. (I had to raise the minimum version of php-cs-fixer to work around a notice that was causing the step to fail, and I had to add prophecy to get tests to run).
• I tried to add a step to cache Composer dependencies, but I couldn't get it to work, and I ran out of time to keep tinkering. It should be okay without it, but caching would be a nice bonus. Feel free to borrow from https://github.com/vufind-org/vufind/blob/dev/.github/workflows/ci.yaml#L58-L67 -- I'm not sure why I couldn't get this to work in the context of the current project.
• I dropped testing support for pre-7.4 PHP versions since they're all long past EOL. I added entries for PHP 8.0 and 8.1 but commented them out for now because they're not going to pass until we finish #110.