composer.json: Add 2.1.x-dev alias for pico-2.1 branch
composer.json: Add 2.1.x-dev alias for pico-2.1 branch

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