Bump Travis CI to use golang 1.13.x (#3262)
Bump Travis CI to use golang 1.13.x (#3262)

Automatically submitted.
chrisohaver/coredns,johnbelamaric/coredns,johnbelamaric/coredns,miekg/coredns,yongtang/coredns,miekg/coredns,coredns/coredns