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

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