.travis: install correct version of ruby
.travis: install correct version of ruby

tgunr/factor,factor/factor,AlexIljin/factor,mrjbq7/factor,AlexIljin/factor,factor/factor,tgunr/factor,mrjbq7/factor,AlexIljin/factor,bpollack/factor,bpollack/factor,AlexIljin/factor,AlexIljin/factor,tgunr/factor,factor/factor,bpollack/factor,AlexIljin/factor,tgunr/factor,tgunr/factor,tgunr/factor,bpollack/factor,factor/factor,mrjbq7/factor,mrjbq7/factor,factor/factor,mrjbq7/factor,factor/factor,bpollack/factor,mrjbq7/factor,AlexIljin/factor,bpollack/factor,bpollack/factor