rename slot_mapping() to slots_mappings() RasaHQ/roadmap#280
rename slot_mapping() to slots_mappings() RasaHQ/roadmap#280

RasaHQ/rasa_core,RasaHQ/rasa_nlu,RasaHQ/rasa_nlu,RasaHQ/rasa_nlu,RasaHQ/rasa_core,RasaHQ/rasa_core