Revert "Update composer.json: Use Symfony YAML 3.1 and later"
Revert "Update composer.json: Use Symfony YAML 3.1 and later"

Symfony YAML >= 3.0 requires PHP >= 5.5.9 and we're currently not planning to raise Pico's PHP requirement.

This reverts commit f251bc83ec995d05e0e33140df22a451c0cfe4af.

PhrozenByte/Pico,TakamiChie/Pico,picocms/Pico,TakamiChie/Pico,PhrozenByte/Pico,TakamiChie/Pico,picocms/Pico,PhrozenByte/Pico,TakamiChie/Pico,picocms/Pico