travis: update go version
travis: update go version

1.9.x match latest patch version(1.9.2 today)
kubernetes/minikube,dalehamel/minikube,aaron-prindle/minikube,dlorenc/minikube,kubernetes/minikube,warmchang/minikube,aaron-prindle/minikube,dlorenc/minikube,r2d4/minikube,kubernetes/minikube,dalehamel/minikube,kubernetes/minikube,dims/minikube,dlorenc/minikube,aaron-prindle/minikube,dalehamel/minikube,warmchang/minikube,warmchang/minikube,warmchang/minikube,dlorenc/minikube,warmchang/minikube,dims/minikube,kubernetes/minikube,dlorenc/minikube,r2d4/minikube,dlorenc/minikube,dlorenc/minikube,dims/minikube,dlorenc/minikube,warmchang/minikube,kubernetes/minikube,r2d4/minikube,kubernetes/minikube,warmchang/minikube