Fix missing dependency zendframework/zend-stdlib

[JurJean]

The code depends on zendframework/zend-stdlib, which is a dependency of a dev dependency. Please either remove the dependency or add zendframework/zend-stdlib to require of composer.json

[Vinceveve]

Done, thank you for the advice :)

[jvdlaar]

The library is now added to the dev dependencies, but it actually is a dependency. The Writer class uses use Zend\Stdlib\SplQueue;

[Vinceveve]

I've just removed all dependencies on Zend Stdlib.