Use GitHub actions for continuous integration (CI)

Bye bye Travis CI, you've served us well.

Update PHPUnit configuration schema for PHPUnit 9.3

Add full core team to the license

Added the full core team in order of joining the team

Run tests on PHPUnit 9 and clean up test suite

Forward compatibility with react/promise 3

Run tests on PHP 7.4 and simplify test matrix

Add .gitattributes to exclude dev files from exports

Test PHP 7.2 and 7.3 on Travis

Check if the function is declared before declaring it.

Update README.md

typo

Improve memory consumption by cleaning up garbage references to pending promise without canceller

Improve memory consumption by cleaning up garbage references

Improve memory consumption by cleaning up unneeded references

Fix test suite forward compatibility with upcoming EventLoop releases

Forward compatiblity with PHPUnit 6

README improvements