Update travis to use 1.15.7 go version.
Update travis to use 1.15.7 go version.

etcd-io/etcd,coreos/etcd,ajityagaty/etcd,mqliang/etcd,rnaveiras/etcd,etcd-io/etcd,rnaveiras/etcd,etcd-io/etcd,rnaveiras/etcd,mqliang/etcd,mqliang/etcd,gyuho/etcd,ajityagaty/etcd,mkumatag/etcd,ajityagaty/etcd,gyuho/etcd,mkumatag/etcd,mkumatag/etcd,coreos/etcd