chore: support go1.13 (#2038)

* chore: support go1.13

* chore: remove env var for go1.13

.travis.yml

@@ -8,6 +8,7 @@ matrix:
     env: GO111MODULE=on
   - go: 1.12.x
     env: GO111MODULE=on
+  - go: 1.13.x
   - go: master
     env: GO111MODULE=on