Cosmetic changes

I forgot quite a few GPL references in my previous release, thus making its licensing ambiguous. Not anymore.

Additionally, I dropped Travis CI config and replaced it with a GitHub Workflows action and updated the PHP versions used during build.

Use MIT license for the library

Change license to MIT

I'm not sure why I chose GPL initially, probably because that's what the C# library was licensed under, and I considered my library a port.
However, I believe I have modified this library significantly enough for it no longer to be considered a simple and straightforward port, and I always preferred more liberal licenses.

Initial release

v1.0

Update README.md