Compatibility with JsonSerializable::jsonSerialize(): mixed

fix: Don't retry jobs infinitely with PredisEngine. Closes #71

fix: correct some code quality issues

fix readme php version

Merge pull request #66 from jeremyharris/patch-1

Added note about redis engines not supporting delay nor expires_in

Merge pull request #64 from felixkempf/patch-1

Update composer.json

Merge pull request #56 from LubosRemplik/workermaxruntime

Implementing runtime and maxRuntime for a worker

Merge pull request #54 from cleptric/master

PdoEngine - Reject expired jobs from the queue

Merge pull request #48 from josegonzalez/fix-mysql-attempts

Fix attempts handling for PDOEngine-based Engines

Merge pull request #44 from scherersoftware/master

Implement attempts functionality for PDO-backend engines