improve Vumi handler, use new USSDSession methods
improve Vumi handler, use new USSDSession methods

tsotetsi/textily-web,pulilab/rapidpro,pulilab/rapidpro,pulilab/rapidpro,tsotetsi/textily-web,tsotetsi/textily-web,tsotetsi/textily-web,pulilab/rapidpro,tsotetsi/textily-web,pulilab/rapidpro