Run travis with current version.

.travis.yml

@@ -4,6 +4,7 @@ dist: xenial
 go:
   - 1.11.x
   - 1.12.x
+  - 1.13.x
 before_install:
 - sudo sysctl net.ipv6.conf.lo.disable_ipv6=0
 - go get github.com/mattn/goveralls